Travis: Revert to optional MYSQL_{BUILD_CC,BUILD_CXX,COMPILER_LAUNCHER}

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>

debian/rules

@@ -84,6 +84,10 @@ endif
 	    -DCOMPILATION_COMMENT="mariadb.org binary distribution" \
 	    -DMYSQL_SERVER_SUFFIX="-$(DEBVERSION)" \
 	    -DSYSTEM_TYPE="debian-$(DEB_BUILD_GNU_SYSTEM)" \
+	    $${MYSQL_BUILD_CXX:+-DCMAKE_CXX_COMPILER=$${MYSQL_BUILD_CXX}} \
+	    $${MYSQL_BUILD_CC:+-DCMAKE_C_COMPILER=$${MYSQL_BUILD_CC}} \
+	    $${MYSQL_COMPILER_LAUNCHER:+-DCMAKE_CXX_COMPILER_LAUNCHER=${MYSQL_COMPILER_LAUNCHER}} \
+	    $${MYSQL_COMPILER_LAUNCHER:+-DCMAKE_C_COMPILER_LAUNCHER=${MYSQL_COMPILER_LAUNCHER}} \
 	    -DCMAKE_SYSTEM_PROCESSOR=$(DEB_BUILD_ARCH) \
 	    -DBUILD_CONFIG=mysql_release \
 	    -DINSTALL_LIBDIR=lib/$(DEB_HOST_MULTIARCH) \