cmake: Move ccache setup to another file

cmake/Modules/ccache.cmake

@@ -0,0 +1,13 @@
+# Using ccache greatly improves the speed of our CI builds, let's enable for all.
+# Without this, our CI can't use ccache for clang, for some reason.
+find_program(CCACHE_FOUND ccache)
+if(CCACHE_FOUND)
+    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_FOUND})
+    set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ${CCACHE_FOUND})
+
+    # ccache uses -I when compiling without preprocessor, which makes clang complain.
+    if("${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")
+    endif()
+endif(CCACHE_FOUND)