MDEV-8551 compilation fails with 10.1.6

Restore changes that were lost in a merge. Originally from

commit 66fd45af
Author: Sergei Golubchik <serg@mariadb.org>
Date:   Mon Jun 8 21:06:56 2015 +0200

    MDEV-7398 mysqld segfaults on FreeBSD 10.1 i386 when built with clang 3.4

storage/mroonga/vendor/groonga/CMakeLists.txt

@@ -143,7 +143,7 @@ if(CMAKE_COMPILER_IS_GNUCC)
 endif()
 
 
-if(CMAKE_COMPILER_IS_GNUCXX)
+if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLANGCXX)
   MY_CHECK_AND_SET_COMPILER_FLAG("-Wall")
   MY_CHECK_AND_SET_COMPILER_FLAG("-Wextra")
   MY_CHECK_AND_SET_COMPILER_FLAG("-Wno-unused-but-set-variable")
@@ -176,7 +176,7 @@ endif()
 
 option(GRN_WITH_DEBUG "enable debug build." OFF)
 if(GRN_WITH_DEBUG)
-  if(CMAKE_COMPILER_IS_GNUCXX)
+  if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLANGCXX)
     set(GRN_C_COMPILE_FLAGS "${GRN_C_COMPILE_FLAGS} -g3 -O0")
     set(GRN_CXX_COMPILE_FLAGS "${GRN_CXX_COMPILE_FLAGS} -g3 -O0")
   endif()