I'm working with the refrence implementation. I'm using version 1.0r9. This is the steps I'm following to set it up:
>./configure --with-threads
Seems to work fine, then:
> make
Things seem to be ok for a while then I get this error:
gcc -I. -I../../include -I./../../include -D__USE_FIXED_PROTOTYPES__ -D_THREAD_SAFE -DHAVE_SETUPTERM -DSOCKS -DINCLUDE_PROTOTYPES -DKLUDGELINEMODE -c ./tn3270.c
gcc -I. -I../../include -I./../../include -D__USE_FIXED_PROTOTYPES__ -D_THREAD_SAFE -DHAVE_SETUPTERM -DSOCKS -DINCLUDE_PROTOTYPES -DKLUDGELINEMODE -c ./utilities.c
make[2]: Target `all' not remade because of errors.
make[2]: Leaving directory `/usr/local/src/socks5-v1.0r9/clients/telnet'
make[1]: *** [all] Error 2
make[1]: Leaving directory `/usr/local/src/socks5-v1.0r9/clients'
make: *** [clients] Error 2

Finally I try to install:
> make install > ~/socks5.log
I get this:
In file included from ./ftp.c:51:
/usr/lib/gcc/i386-redhat-linux/3.4.3/include/varargs.h:4:2: #error "GCC no longer implements <varargs.h>."
/usr/lib/gcc/i386-redhat-linux/3.4.3/include/varargs.h:5:2: #error "Revise your code to use <stdarg.h>."
./ftp.c:244: error: syntax error before "va_dcl"
./ftp.c:244: error: syntax error before '{' token
./ftp.c:250: error: redefinition of 'abrtflag'
./ftp.c:79: error: previous definition of 'abrtflag' was here
./ftp.c:250: warning: data definition has no type or storage class
./ftp.c:251: error: syntax error before "if"
./ftp.c:254: warning: parameter names (without types) in function declaration
./ftp.c:254: warning: data definition has no type or storage class
./ftp.c:255: error: conflicting types for 'fmt'
./ftp.c:246: error: previous declaration of 'fmt' was here
./ftp.c:255: error: `ap' undeclared here (not in a function)
./ftp.c:255: error: syntax error before "char"
./ftp.c:260: warning: parameter names (without types) in function declaration
./ftp.c:260: warning: data definition has no type or storage class
./ftp.c:262: error: syntax error before string constant
./ftp.c:262: error: conflicting types for 'printf'
./ftp.c:262: note: a parameter list with an ellipsis can't match an empty parameter name list declaration
./ftp.c:262: error: conflicting types for 'printf'
./ftp.c:262: note: a parameter list with an ellipsis can't match an empty parameter name list declaration
./ftp.c:262: warning: data definition has no type or storage class
./ftp.c:263: warning: parameter names (without types) in function declaration
./ftp.c:263: warning: data definition has no type or storage class
./ftp.c:264: error: syntax error before '}' token
./ftp.c:268: warning: data definition has no type or storage class
./ftp.c:269: error: syntax error before "return"
./ftp.c:272: error: conflicting types for 'oldintr'
./ftp.c:248: error: previous declaration of 'oldintr' was here
./ftp.c:272: warning: initialization makes integer from pointer without a cast
./ftp.c:272: error: initializer element is not constant
./ftp.c:272: warning: data definition has no type or storage class
./ftp.c:274: warning: parameter names (without types) in function declaration
./ftp.c:274: warning: data definition has no type or storage class
./ftp.c:275: error: redefinition of 'fmt'
./ftp.c:255: error: previous definition of 'fmt' was here
./ftp.c:275: error: redefinition of 'fmt'
./ftp.c:255: error: previous definition of 'fmt' was here
./ftp.c:275: error: syntax error before "char"
./ftp.c:276: warning: parameter names (without types) in function declaration
./ftp.c:276: error: conflicting types for 'SOCKSvfprintf'
./ftp.c:276: note: a parameter list with an ellipsis can't match an empty parameter name list declaration
../../include/socks.h:38: error: previous declaration of 'SOCKSvfprintf' was here
./ftp.c:276: error: conflicting types for 'SOCKSvfprintf'
./ftp.c:276: note: a parameter list with an ellipsis can't match an empty parameter name list declaration
../../include/socks.h:38: error: previous declaration of 'SOCKSvfprintf' was here
./ftp.c:276: warning: data definition has no type or storage class
./ftp.c:277: warning: parameter names (without types) in function declaration
./ftp.c:277: warning: data definition has no type or storage class
./ftp.c:279: warning: parameter names (without types) in function declaration
./ftp.c:279: warning: data definition has no type or storage class
./ftp.c:281: warning: data definition has no type or storage class
./ftp.c:282: warning: passing arg 1 of `strcmp' makes pointer from integer without a cast
./ftp.c:282: error: initializer element is not constant
./ftp.c:282: warning: data definition has no type or storage class
./ftp.c:284: error: syntax error before "if"
./ftp.c:285: error: syntax error before numeric constant
./ftp.c:285: error: conflicting types for 'Signal'
../../include/sigfix.h:20: error: previous definition of 'Signal' was here
./ftp.c:285: warning: data definition has no type or storage class
make[2]: *** [ftp.o] Error 1
make[2]: Target `all' not remade because of errors.
In file included from ./commands.c:86:
/usr/lib/gcc/i386-redhat-linux/3.4.3/include/varargs.h:4:2: #error "GCC no longer implements <varargs.h>."
/usr/lib/gcc/i386-redhat-linux/3.4.3/include/varargs.h:5:2: #error "Revise your code to use <stdarg.h>."
./commands.c:2096: error: syntax error before "va_dcl"
./commands.c:2097: error: syntax error before '{' token
./commands.c:2104: warning: parameter names (without types) in function declaration
./commands.c:2104: warning: data definition has no type or storage class
./commands.c:2105: error: conflicting types for 'routine'
./commands.c:2100: error: previous declaration of 'routine' was here
./commands.c:2105: error: `ap' undeclared here (not in a function)
./commands.c:2105: error: syntax error before "intrtn_t"
./commands.c:2109: warning: parameter names (without types) in function declaration
./commands.c:2109: warning: data definition has no type or storage class
./commands.c:2110: error: syntax error before "return"
./commands.c:125: warning: 'call' used but never defined
make[2]: *** [commands.o] Error 1
make[2]: Target `all' not remade because of errors.
make[1]: *** [all] Error 2
make: *** [clients] Error 2

Not sure what to do, help is appreciated . Running CentOS 4.1. Also feel free to suggest other proxy software...