GCC: Fix building dav1d by defining _GNU_SOURCE

dav1d is build with C99 which does not define _GNU_SOURCE by
default. However, GNU extensions are used within dav1d,
e.g. RTLD_DEFAULT. clang build succeeds, because _GNU_SOURCE
is defined globally.

Bug: 819294
Change-Id: I1c6e024421c8c8de0b44f0d91dc0b96ef967e712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010760
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735044}

third_party/dav1d/BUILD.gn

@@ -113,6 +113,11 @@ if (is_win) {
   if (is_mac) {
     dav1d_copts += [ "-D_DARWIN_C_SOURCE" ]
   }
+  if (is_linux || is_android || current_os == "aix") {
+    if (!is_clang) {
+      dav1d_copts += [ "-D_GNU_SOURCE" ]
+    }
+  }
 }
 
 if (enable_nasm) {