fix: Disable warnings for GNU extensions on Clang

Some versions of Clang enabled '-Wgnu' in '-Wall', since we rely on
GNUisms in the code this results in numerous errors. Check if the
compiler accepts '-Wno-gnu' to disable those warnings.

Change-Id: I9d1126744e427a6cf7c18e219cae5431227a43c0
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>