ADD: enable ccache when detected

Summary:
-Dramatically and safely improve the speed of C/C++ recompiles by utilizing ccache when detected.  Very helpful after a 'make clean', when changing between release and debug builds, or when maintaining builds in multiple directories.  Caches can also be shared locally or via NFS for environments with multiple developers.

ccache: https://ccache.samba.org/
Closes #7119

Reviewed By: Orvid

Differential Revision: D3366778

fbshipit-source-id: dfc4785e5692401b783d092830f23e4747850cca

CMakeLists.txt

@@ -41,6 +41,14 @@ IF(NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
   MESSAGE(FATAL_ERROR "HHVM requires a 64bit OS")
 ENDIF()
 
+# If available, configure CCache
+find_program(CCACHE_FOUND ccache)
+if(CCACHE_FOUND)
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
+  set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
+  message(STATUS "Found ccache, enabling")
+endif(CCACHE_FOUND)
+
 # 3rd party library
 IF(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/third-party/CMakeLists.txt")
   MESSAGE(FATAL_ERROR "third-party/CMakeLists.txt missing. "