LinuxQuestions.org - [SOLVED] Undefined reference to symbol 'pthread_once@@GLIBC

I am trying to install this program and everything was going good, 97% done, when I get this error:

Code:

/usr/lib64/gcc/x86_64-slackware-linux/4.7.1/../../../../x86_64-slackware-linux/bin/ld: ../common/pb/libcockatrice_protocol.a(admin_commands.pb.cc.o): undefined reference to symbol 'pthread_once@@GLIBC_2.2.5'

/usr/lib64/gcc/x86_64-slackware-linux/4.7.1/../../../../x86_64-slackware-linux/bin/ld: note: 'pthread_once@@GLIBC_2.2.5' is defined in DSO /lib64/libpthread.so.0 so try adding it to the linker command line

/lib64/libpthread.so.0: could not read symbols: Invalid operation

It says to add /lib64/libpthread.so.0 to the link, but I'm not sure how? Any advice would be much appreciated.

Edit: I managed to fix it with a big of a brute force solution. I found the file where the flags for the cmake command were and inserted "-L/lib64 -lpthread" after all the C flag lists.