Remove hideous warnings with clang-based builds.

Using both ccache and clang ends up in tons of spammy warnings
and disables Clang's colored output on errors. For more details
see:

   http://petereisentraut.blogspot.fr/2011/05/ccache-and-clang.html

This patch fixes these issues as recommended by the article.

Change-Id: Icbfd1ed5b24904887a8796575601d471039ee450
1 file changed