cmake: Only set -D_FORTIFY_SOURCE=2 if we have optimizations.

cmake/Modules/DefineCompilerFlags.cmake

@@ -26,10 +26,13 @@ if (UNIX AND NOT WIN32)
             set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fstack-protector")
         endif (WITH_STACK_PROTECTOR)
 
-        check_c_compiler_flag("-D_FORTIFY_SOURCE=2" WITH_FORTIFY_SOURCE)
-        if (WITH_FORTIFY_SOURCE)
-            set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_FORTIFY_SOURCE=2")
-        endif (WITH_FORTIFY_SOURCE)
+        string(TOLOWER ${CMAKE_BUILD_TYPE} CMAKE_BUILD_TYPE_LOWER)
+        if (NOT CMAKE_BUILD_TYPE_LOWER MATCHES debug)
+            check_c_compiler_flag("-D_FORTIFY_SOURCE=2" WITH_FORTIFY_SOURCE)
+            if (WITH_FORTIFY_SOURCE)
+                set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_FORTIFY_SOURCE=2")
+            endif (WITH_FORTIFY_SOURCE)
+        endif()
     endif (${CMAKE_C_COMPILER_ID} MATCHES "(GNU|Clang)")
 
     #