asctime: check for standards compliance (Solaris support)

asctime: check for standards compliance (Solaris support)

Add checks to "configure" to see whether _POSIX_PTHREAD_SEMANTICS needs
to be defined to get the right number of arguments in the prototypes for
asctime_r().  Solaris' default implementation conforms to POSIX.1c
Draft 6, rather than the final POSIX.1c spec.  The standards-compliant
version can be used by defining _POSIX_PTHREAD_SEMANTICS.

This change also adds the file "compat/check_asctime.c", which
configure uses to perform its check, and modifies compat/compat.h to
define _POSIX_PTHREAD_SEMANTICS if configure detected it was needed.

Signed-off-by: Vladimir Marek <vlmarek@volny.cz>