tools: don't unnecessarily redefine _GNU_SOURCE in config.h

Makes it easier to reuse this code in other projects.

config.h

@@ -1,7 +1,9 @@
 /* Generated by CCAN configurator */
 #ifndef CCAN_CONFIG_H
 #define CCAN_CONFIG_H
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE /* Always use GNU extensions. */
+#endif
 #define CCAN_COMPILER "cc"
 #define CCAN_CFLAGS "-g -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -DCCAN_STR_DEBUG=1"
 

tools/configurator/configurator.c

@@ -394,7 +394,9 @@ int main(int argc, const char *argv[])
 	printf("/* Generated by CCAN configurator */\n"
 	       "#ifndef CCAN_CONFIG_H\n"
 	       "#define CCAN_CONFIG_H\n");
+	printf("#ifndef _GNU_SOURCE\n");
 	printf("#define _GNU_SOURCE /* Always use GNU extensions. */\n");
+	printf("#endif\n");
 	printf("#define CCAN_COMPILER \"%s\"\n", argv[1]);
 	printf("#define CCAN_CFLAGS \"%s\"\n\n", cmd + strlen(argv[1]) + 1);
 	for (i = 0; i < sizeof(tests)/sizeof(tests[0]); i++)