mdbx-cmake: avoid using check_cxx_source_compiles().

More for https://github.com/erthink/libmdbx/issues/111

Change-Id: Icbe1fb3c4ed8e22da7fa7e95d2c8f2c302e77d82

cmake/compiler.cmake

@@ -228,7 +228,7 @@ else()
 
   # Check for an omp support
   set(CMAKE_REQUIRED_FLAGS "-fopenmp -Werror")
-  check_cxx_source_compiles("int main(void) {
+  check_c_source_compiles("int main(void) {
     #pragma omp parallel
     return 0;
     }" HAVE_OPENMP)