Set _GNU_SOURCE

This is set implicitly by some compiler invocations.

Respect $CC as the Makefile does

Author: eradman

Makefile.linux

@@ -1,4 +1,4 @@
-CPPFLAGS += -D_LINUX_PORT -Imissing
+CPPFLAGS += -D_GNU_SOURCE -D_LINUX_PORT -Imissing
 MANPREFIX ?= ${PREFIX}/share/man
 EXTRA_SRC = missing/kqueue_inotify.c
 

Makefile.linux-compat

@@ -1,4 +1,4 @@
-CPPFLAGS += -D_LINUX_PORT -Imissing
+CPPFLAGS += -D_GNU_SOURCE -D_LINUX_PORT -Imissing
 MANPREFIX ?= ${PREFIX}/share/man
 EXTRA_SRC = missing/strlcpy.c missing/kqueue_inotify.c
 

configure

@@ -1,7 +1,9 @@
 #!/bin/sh
 
 test_libc_features() {
-	2>/dev/null gcc -xc - <<-EOF
+	CFLAGS="-D_GNU_SOURCE"
+	: ${CC:=cc}
+	2>/dev/null $CC -xc $CFLAGS -o /dev/null - <<-EOF
 	#include <string.h>
 	int main(void) {
 		char dst[4];