CMake: Fix CCache support when already using _LAUNCHER

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

CMake/CCache.cmake

@@ -1,10 +1,19 @@
 find_program(CCACHE_BIN NAMES ccache sccache)
 if(CCACHE_BIN)
-    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_BIN})
+    # Official ccache recommendation is to set CMAKE_C(XX)_COMPILER_LAUNCHER
+    if (NOT CMAKE_C_COMPILER_LAUNCHER MATCHES "ccache")
+        list(INSERT CMAKE_C_COMPILER_LAUNCHER 0 "${CCACHE_BIN}")
+        # ccache uses -I when compiling without preprocessor, which makes clang complain.
+        if("${CMAKE_C_COMPILER_ID}" STREQUAL "Clang")
+            set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Qunused-arguments -fcolor-diagnostics")
+        endif()
+    endif()
 
-    # ccache uses -I when compiling without preprocessor, which makes clang complain.
-    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
-        set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Qunused-arguments -fcolor-diagnostics")
-        set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Qunused-arguments -fcolor-diagnostics")
+    if (NOT CMAKE_CXX_COMPILER_LAUNCHER MATCHES "ccache")
+        list(INSERT CMAKE_CXX_COMPILER_LAUNCHER 0 "${CCACHE_BIN}")
+        # ccache uses -I when compiling without preprocessor, which makes clang complain.
+        if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+            set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Qunused-arguments -fcolor-diagnostics")
+        endif()
     endif()
 endif()