Fix warnings when building with gcc 4.8

This patch adds some warning flags and feature detection from upstream
javascriptcore which gets rid of warnings on COMPILE_ASSERT.

My upstream patch: http://trac.webkit.org/changeset/146993



Change-Id: Idd38ef39b6f171a1c90763b2d9d000a4d00374ba
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>