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

To facilitate multiple build types in Travis, all supported
by MYSQL_COMPILER_LAUNCHER (usually ccache).

As noted by @ottok in MDEV-10752 e14bdcb no longer influences the compiler used during the build due to other changes in the build system. An environment variable CC/CXX didn't propagate to the build so a variant of the previous MYSQL_BUILD_CC/MYSQL_BUILD_CXX was introduced using the cmake documented way of specifying C/C++ compilers. Added MYSQL_COMPILER_LAUNCHER which can be set to ccache to facilitate any compiler.