-D_REENTRANT -D_THREAD_SAFE -D_POSIX_PTHREAD_SEMANTICS
for all ports. It can't hurt if they are not supported, but it makes
our job easier for porting.
Should fix Darwin compile and other platforms without mucking with the
thread detection code.
Allow additional thread flags to be added via port templates.
Change thread flag names to PTHREAD_CFLAGS and PTHREAD_LIBS to match new
configure script.