spawn-scm-0.rockspec: define _GNU_SOURCE

Currently glibc hides _POSIX_SPAWN_SETSID behind it.
This also gets us POSIX_SPAWN_USEVFORK.

Author: daurnimator

spawn-scm-0.rockspec

@@ -20,7 +20,10 @@ build = {
 	modules = {
 		["spawn.init"] = "spawn/init.lua";
 		["spawn.posix"] = {
-			defines = { "_POSIX_C_SOURCE=200809L" };
+			defines = {
+				"_POSIX_C_SOURCE=200809L";
+				"_GNU_SOURCE";
+			};
 			incdirs = { "vendor/compat-5.3/c-api/" };
 			sources = {
 				"spawn/posix.c";