Define is_gcc and enable the warning flag if it is true.
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>

Bug: angleproject:4486
Change-Id: Ie3271cad5cd7de7b45a48d520eeadfe85e742254
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2690954
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>