cmake: Use -fstack-protector-strong if possible

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

CompilerChecks.cmake

@@ -63,10 +63,15 @@ if (UNIX)
         endif()
     endif()
 
-    check_c_compiler_flag_ssp("-fstack-protector" WITH_STACK_PROTECTOR)
-    if (WITH_STACK_PROTECTOR)
-        list(APPEND SUPPORTED_COMPILER_FLAGS "-fstack-protector")
-    endif()
+    check_c_compiler_flag_ssp("-fstack-protector-strong" WITH_STACK_PROTECTOR_STRONG)
+    if (WITH_STACK_PROTECTOR_STRONG)
+        list(APPEND SUPPORTED_COMPILER_FLAGS "-fstack-protector-strong")
+    else (WITH_STACK_PROTECTOR_STRONG)
+        check_c_compiler_flag_ssp("-fstack-protector" WITH_STACK_PROTECTOR)
+        if (WITH_STACK_PROTECTOR)
+            list(APPEND SUPPORTED_COMPILER_FLAGS "-fstack-protector")
+        endif()
+    endif (WITH_STACK_PROTECTOR_STRONG)
 
     if (PICKY_DEVELOPER)
         add_c_compiler_flag("-Wno-error=deprecated-declarations" SUPPORTED_COMPILER_FLAGS)