Fix cmake breakage (thanks Marcus)

My recent change broke the sdlcheck.cmake by not escaping some quotes properly.
Thanks to Marcus von Appen for pointing it out and providing the patch.

cmake/sdlchecks.cmake

@@ -19,7 +19,7 @@ macro(CheckDLOPEN)
     check_c_source_compiles("
        #include <dlfcn.h>
        int main(int argc, char **argv) {
-         void *handle = dlopen("", RTLD_NOW);
+         void *handle = dlopen(\"\", RTLD_NOW);
          const char *loaderror = (char *) dlerror();
        }" HAVE_DLOPEN)
     set(CMAKE_REQUIRED_LIBRARIES)