cmake: adjust ccache messages given ut's now optional

cmake/CMakeLists.txt

@@ -182,7 +182,7 @@ endif()
 if(USE_CCACHE)
     find_program(CCACHE_FOUND ccache)
     if(CCACHE_FOUND)
-        message(STATUS "`ccache` is found, using it as a compiler launcher.")
+        message(STATUS "`USE_CCACHE` is enabled and `ccache` is found, using it as a compiler launcher.")
         set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE "${CCACHE_FOUND}")
         if(MSVC)
             # By default Visual Studio generators will use /Zi which is not compatible
@@ -202,7 +202,7 @@ if(USE_CCACHE)
         endif()
     endif()
 else()
-    message(STATUS "ccache usage disabled via USE_CCACHE=OFF")
+    message(STATUS "ccache usage is disabled, set `USE_CCACHE=ON` to enable it.")
 endif()
 
 if(MSVC AND MSVC_PARALLEL_BUILD)