Fails to link with libgflags_nothreads.so: undefined reference to `pthread_rwlock_unlock' #14
Description
Building with gcc 6, make fails with:
Linking guetzli
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/libgflags_nothreads.so: undefined reference to `pthread_rwlock_unlock'
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/libgflags_nothreads.so: undefined reference to `pthread_rwlock_init'
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/libgflags_nothreads.so: undefined reference to `pthread_rwlock_destroy'
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/libgflags_nothreads.so: undefined reference to `pthread_rwlock_wrlock'
collect2: error: ld returned 1 exit status
guetzli.make:103: recipe for target 'bin/Release/guetzli' failed
make[1]: *** [bin/Release/guetzli] Error 1
Makefile:28: recipe for target 'guetzli' failed
make: *** [guetzli] Error 2