This is a bug with imake (xmkmf). The symbol
DefaultGcc2i386Opt is supposed to be translated to a series of options before compilation. If xmkmf ignores it and it's not translated, gcc thinks it's being asked to compile two files at once, hence the error.
One workaround is to add the following to
/usr/X11R6/lib/X11/config/linux.cf:
Code:
#ifndef DefaultGcc2i386Opt
# define DefaultGcc2i386Opt -O3
#endif