Philip Withnall authored Nov 01, 2013 and Olivier Crête committed Jan 07, 2014

Do not unconditionally enable -Werror, since that breaks the build for
people who have stricter compilation warnings enabled.

Instead, add an --enable-compile-warnings configure flag which enables
-Werror when passed as --enable-compile-warnings=error. This mimics the
flag in gnome-common, for consistency. But we enable errors by default
on non-released versions.