The official ccache documentation[1] recommends to set
`CMAKE_C(XX)_COMPILER_LAUNCHER` to ccache to enable ccache.
These also work as envionment variables (supported by CMake itself).
However, using these instructions generates the following error during
building:
ccache: error: Recursive invocation (the name of the ccache binary must be "ccache")
This is because Dolphin adds an additional command ccache layer (ccache
ccache compiler ...).
This fixes that issue by checking for `CMAKE_C(XX)_COMPILER_LAUNCHER`
before inserting our own. Also, use `CMAKE_C(XX)_COMPILER_LAUNCHER`
to add ccache because the CMake docs discourages the use of
`RULE_LAUNCH_COMPILE` in favour of `CMAKE_C(XX)_COMPILER_LAUNCHER`.
[1]: https://github.com/ccache/ccache/wiki/CMake
