linux: Add _GNU_SOURCE definition.

This fixes linux-x64 builds so that the compiler can find
readlink() and strtok_r().

Change-Id: I9f2dc678e66a7c80045d2c8d561fa01dd1ebbd37

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 4ea3309..394f491 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -83,8 +83,11 @@
     cflags = [ "/wd4201" ]
   }
   if (is_linux || is_chromeos) {
-    # assume secure_getenv() is available
     defines += [
+      # Enables `readlink()` and `strtok_r()`.
+      "_GNU_SOURCE",
+
+      # assume secure_getenv() is available
       "HAVE_SECURE_GETENV",
       "LOADER_ENABLE_LINUX_SORT",
     ]