respect compiler and flags from system
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -110,7 +110,7 @@ else()
   set(GRN_BUNDLED TRUE)
 endif()
 
-if(NOT GRN_BUNDLED)
+if(NOT GRN_BUNDLED AND GRN_CCACHE)
   find_program(CCACHE ccache)
   if(CCACHE)
     if(NOT CMAKE_C_COMPILER_LAUNCHER)
@@ -239,10 +239,6 @@ if(GRN_C_COMPILER_GNU_LIKE)
   set(GRN_C_COMPILE_FLAGS "${GRN_C_COMPILE_FLAGS} $<$<CONFIG:Debug>:-g3 -O0>")
   set(GRN_CXX_COMPILE_FLAGS
       "${GRN_CXX_COMPILE_FLAGS} $<$<CONFIG:Debug>:-g3 -O0>")
-  set(GRN_C_COMPILE_FLAGS
-      "${GRN_C_COMPILE_FLAGS} $<$<CONFIG:RelWithDebInfo>:-g3>")
-  set(GRN_CXX_COMPILE_FLAGS
-      "${GRN_CXX_COMPILE_FLAGS} $<$<CONFIG:RelWithDebInfo>:-g3>")
 endif()
 
 # /Ob1 is used with RelWithDebInfo by default but it's slower than /Ob2 which is