pthread.buildlink3.mk into a separate file, pthread.builtin.mk,
that is handled using the usual builtin.mk logic.
(2) If pthread.buildlink3.mk is included by a package Makefile, then
automatically add the necessary compiler and linker flags to
compile and link pthread-enabled/reentrant code. For native
pthreads, this means passing -pthread to the compiler and linker.
For the userland pthread replacement, we pass -D_REENTRANT and
-lpthread instead.
(3) Add PTHREAD_{CFLAGS,LDFLAGS,LIBS} in both CONFIGURE_ENV and MAKE_ENV
when pthread.buildlink3.mk is included so that the configure and
build processes can use these values. Remove these definitions
from bsd.pkg.mk since PTHREAD_* variables are all declared within
pthread.buildlink3.mk.
XXX For now, PTHREAD_LDFLAGS is a superset of PTHREAD_LIBS until
XXX packages that use use PTHREAD_LDFLAGS can be fixed.
