To facilitate multiple build types in Travis, the environment variable
MYSQL_{BUILD_CC,BUILD_CXX} will be passed to cmake if used. This will
fallback to $CC/$CXX otherwise.

Added MYSQL_COMPILER_LAUNCHER (usually ccache) which isn't supported
until cmake-3.4, which isn't in travis (trusty), but hopefully a later
version can use it or in CI systems other than travis.
Signed-off-by: Daniel Black <daniel.black@au.ibm.com>