package/ccache: do not force colored diagnostics

package/ccache: do not force colored diagnostics

When GCC_COLORS is set, ccache passes '-fdiagnostics-color' to GCC but
this flag requires GCC v4.9 or later. Older versions complain about the
unrecognized command line option.

Using GCC_COLORS in the context of Buildroot is seldom useful, so we
just unexport GCC_COLORS altogether.

Reported-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>