Dtrace cmake fix for clang

When there are GCC-incompatible compiler flags dtrace fails like this:

gcc: error: unrecognized command-line option ‘-fno-limit-debug-info’
gcc: error: unrecognized command-line option ‘-mbranches-within-32B-boundaries’
"gcc .dtrace-temp.3fd6bacf.c" failed
Usage /usr/bin/dtrace [--help] [-h | -G] [-C [-I<Path>]] -s File.d [-o <File>]

Author: midenok

cmake/dtrace.cmake

@@ -125,16 +125,20 @@ FUNCTION(DTRACE_INSTRUMENT target)
         WORKING_DIRECTORY ${objdir}
       )
     ELSEIF(CMAKE_SYSTEM_NAME MATCHES "Linux")
-      # dtrace on Linux runs gcc and uses flags from environment
-      SET(CFLAGS_SAVED $ENV{CFLAGS})
-      SET(ENV{CFLAGS} ${CMAKE_C_FLAGS})
+      IF (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+        # dtrace on Linux runs gcc and uses flags from environment
+        SET(CFLAGS_SAVED $ENV{CFLAGS})
+        SET(ENV{CFLAGS} ${CMAKE_C_FLAGS})
+      ENDIF()
       SET(outfile "${CMAKE_BINARY_DIR}/probes_mysql.o")
       # Systemtap object
       EXECUTE_PROCESS(
         COMMAND ${DTRACE} -G -s ${CMAKE_SOURCE_DIR}/include/probes_mysql.d.base
         -o ${outfile}
         )
-      SET(ENV{CFLAGS} ${CFLAGS_SAVED})
+      IF (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+        SET(ENV{CFLAGS} ${CFLAGS_SAVED})
+      ENDIF()
     ENDIF()
 
     # Do not try to extend the library if we have not built the .o file