Fix building of hwf-x86.c

* src/Makefile.am (AM_CFLAGS): Set to GPG_ERROR_CFLAGS
(AM_CCASFLAGS): Set NOEXECSTACK_FLAGS.

Without that fix we might have used a different gcrypt.h version if we
explicitly requested a newer one. Note that dynamically added modules
use different rules than the static ones (.c.lo: vs. .c.la:).

AM_CCASFLAGS might not be needed right now but may be useful in the
future.