ccache: Allow external setting of CCACHE_COMPILERCHECK variable

This patch allows usage of a custom CCACHE_COMPILERCHECK setting
for the special cases when "content" strategy is not enough to ensure
that results retrieved from the cache are accurate for the current
compiler.

BUG=none
Change-Id: I30be5182cfb36ec30f38b9a2fac89290bd5df910
Signed-off-by: Stefan Stanacar <stefan.stanacar@intel.com>