Unix/macOS: support building with Ccache

This adds an option (WITH_COMPILER_CCACHE) to build using Ccache if it's
found. Makefiles-based, Ninja-based and Xcode generators are supported.

Pass `-DWITH_COMPILER_CCACHE=ON` to cmake to enable Ccache.
Utility option in GNUmakefile is also added: for e.g.,
`make ninja ccache`.

Reviewed By: brecht, ankitm
Differential Revision: https://developer.blender.org/D9665

CMakeLists.txt

@@ -610,6 +610,11 @@ if(WIN32)
 
 endif()
 
+if(UNIX)
+  # See WITH_WINDOWS_SCCACHE for Windows.
+  option(WITH_COMPILER_CCACHE "Use ccache to improve rebuild times (Works with Ninja, Makefiles and Xcode)" OFF)
+endif()
+
 # The following only works with the Ninja generator in CMake >= 3.0.
 if("${CMAKE_GENERATOR}" MATCHES "Ninja")
   option(WITH_NINJA_POOL_JOBS