activate libssp compiler flags for gcc only

In 82c8a1e3 I made "--param ssp-buffer-size=4" one argument instead of
two to fix another problem when passing compiler flags to
CMAKE_REQUIRED_LIBRARIES, clang cannot deal with this and this breaks
the build on mac.

Enable libssp related flags for gcc only, because clang does not support
them anyway. This fixes the build issue on mac.

CMakeLists.txt

@@ -335,17 +335,20 @@ IF(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID STREQUAL Clang)
         SET(MY_C_FLAGS ${MY_C_FLAGS} -fPIC)
     ENDIF()
 
-    # check if SSP flags are supported
     INCLUDE(CheckCXXCompilerFlag)
-    CHECK_CXX_COMPILER_FLAG(-fstack-protector-strong  F_STACK_PROTECTOR_STRONG_FLAG)
 
-    IF(F_STACK_PROTECTOR_STRONG_FLAG)
-        SET(MY_C_FLAGS ${MY_C_FLAGS} -fstack-protector-strong)
+    # check if ssp flags are supported for this version of gcc
+    IF(CMAKE_COMPILER_IS_GNUCXX)
+        CHECK_CXX_COMPILER_FLAG(-fstack-protector-strong  F_STACK_PROTECTOR_STRONG_FLAG)
 
-        CHECK_CXX_COMPILER_FLAG("--param ssp-buffer-size=4" SSP_BUFFER_SIZE_FLAG)
+        IF(F_STACK_PROTECTOR_STRONG_FLAG)
+            SET(MY_C_FLAGS ${MY_C_FLAGS} -fstack-protector-strong)
 
-        IF(SSP_BUFFER_SIZE_FLAG)
-            SET(MY_C_FLAGS ${MY_C_FLAGS} "--param ssp-buffer-size=4")
+            CHECK_CXX_COMPILER_FLAG("--param ssp-buffer-size=4" SSP_BUFFER_SIZE_FLAG)
+
+            IF(SSP_BUFFER_SIZE_FLAG)
+                SET(MY_C_FLAGS ${MY_C_FLAGS} "--param ssp-buffer-size=4")
+            ENDIF()
         ENDIF()
     ENDIF()