Merge pull request #1587 from reyoung/feature/fix_ccache_not_in_path

Fix the way Paddle reference ccache program

cmake/ccache.cmake

 # Use ccache if found ccache program
 
-find_program(CCACHE_FOUND ccache)
+find_program(CCACHE_PATH ccache)
 
-if(CCACHE_FOUND)
+if(CCACHE_PATH)
     message(STATUS "Ccache is founded, use ccache to speed up compile.")
-    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
-    set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
-endif(CCACHE_FOUND)
\ No newline at end of file
+    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PATH})
+    set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ${CCACHE_PATH})
+endif(CCACHE_PATH)