Fix 'pthread_np.h: no such file' compiler error on kFreeBSD

Fix 'pthread_np.h: no such file' compiler error on kFreeBSD
(fix commit 49d7fe7)

Issue #180 (bdwgc).

* include/private/gcconfig.h [(FREEBSD && __GLIBC__ || LINUX
|| PLATFORM_ANDROID) && !NO_PTHREAD_GETATTR_NP]
(HAVE_PTHREAD_GETATTR_NP): New macro.
* include/private/gcconfig.h [FREEBSD && !__GLIBC__
&& !NO_PTHREAD_ATTR_GET_NP] (HAVE_PTHREAD_NP_H,
HAVE_PTHREAD_ATTR_GET_NP): Likewise.
* os_dep.c [!BEOS && !AMIGA && !OS2 && !MSWIN32 && !MSWINCE
&& !CYGWIN32 && !GC_OPENBSD_THREADS] (include pthread.h,
include pthread_np.h, GC_get_main_stack_base): Check presence of
HAVE_PTHREAD_ATTR_GET_NP, HAVE_PTHREAD_GETATTR_NP, HAVE_PTHREAD_NP_H
macros instead of FREEBSD, NO_PTHREAD_ATTR_GET_NP, LINUX,
PLATFORM_ANDROID, NO_PTHREAD_GETATTR_NP.
* os_dep.c [!HAVE_GET_STACK_BASE] (GC_get_stack_base): Check presence
of THREADS, HAVE_PTHREAD_ATTR_GET_NP, HAVE_PTHREAD_GETATTR_NP,
HAVE_PTHREAD_NP_H macros instead of GC_FREEBSD_THREADS,
NO_PTHREAD_ATTR_GET_NP, GC_LINUX_THREADS, PLATFORM_ANDROID,
NO_PTHREAD_GETATTR_NP, FREEBSD.