configure.ac: drop pthread-stubs on FreeBSD

libpthread-stubs >= 0.4 isn't granular enough: checks too few or often unused. For example, libxcb uses

$ pkg info -ql libxcb | grep '.so$' | xargs nm -D | fgrep pthread_ | sort -u
                 U pthread_cond_broadcast
                 U pthread_cond_destroy
                 U pthread_cond_init
                 U pthread_cond_signal
                 U pthread_cond_wait
                 U pthread_mutex_destroy
                 U pthread_mutex_init
                 U pthread_mutex_lock
                 U pthread_mutex_unlock

all of which are stubbed

• https://github.com/DragonFlyBSD/DragonFlyBSD/blob/DragonFly_RELEASE_6_4/lib/libc/gen/_pthread_stubs.c
• https://github.com/freebsd/freebsd-src/blob/releng/13.1/lib/libc/gen/_pthread_stubs.c

but libpthread-stubs itself wants pthread_condattr_*, so adds -pthread which expands to just -lpthread.

CC @manu @valpackett