x11-wm/wlmaker: unbreak build on non-x86 architectures

Upstream defines _POSIX_C_SOURCE to 1, 199309L, 200112L, 200809L in
multiple files, all of which hide SIGTRAP on FreeBSD. Drop POSIX as
BSD (unlike GNU) enable extensions by default.

submodules/libbase/subprocess.c:496:9: error: use of undeclared identifier 'SIGTRAP'
  496 |         BS_ASSERT(rv == subprocess_ptr->pid);
      |         ^
submodules/libbase/assert.h:45:13: note: expanded from macro 'BS_ASSERT'
   45 |             BS_ABORT();                                                 \
      |             ^
submodules/libbase/assert.h:34:28: note: expanded from macro 'BS_ABORT'
   34 | #define BS_ABORT() { raise(SIGTRAP); }
      |                            ^

PR:		276743
Reported by:	pkg-fallout (aarch64)

x11-wm/wlmaker/Makefile

@@ -43,5 +43,8 @@ post-patch:
 		 -e 's/google-chrome/chrome/' \
 		${WRKSRC}/src/dock.c \
 		${WRKSRC}/src/${PORTNAME}.c
+# https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=276743
+	@${GREP} -Flr _POSIX_C_SOURCE ${WRKSRC} | ${XARGS} ${REINPLACE_CMD} \
+		'/_POSIX_C_SOURCE/d'
 
 .include <bsd.port.mk>