GL: suppress an error/warning on MinGW Clang too, not just clang-cl.

Although in this case it seems that it doesn't warn at all, for some
reason, unlike with the same case in Corrade's Debug global state?
Weird.

src/Magnum/GL/Context.cpp

@@ -699,10 +699,10 @@ Context* currentContext = nullptr;
    called from. To avoid #ifdef hell in code below, the currentContext is
    redefined to return a value from this uniqueness-ensuring function. */
 #if defined(CORRADE_TARGET_WINDOWS) && defined(MAGNUM_BUILD_STATIC_UNIQUE_GLOBALS) && !defined(CORRADE_TARGET_WINDOWS_RT)
-/* Clang-CL (and regular Clang as well) complains that the function has a
-   return type incompatible with C. I don't care, I only need an unmangled name
-   to look up later at runtime. */
-#ifdef CORRADE_TARGET_CLANG
+/* MinGW Clang and Clang-CL complains that the function has a return type
+   incompatible with C. I don't care, I only need an unmangled name to look up
+   later at runtime. */
+#if defined(CORRADE_TARGET_CLANG_CL) || (defined(CORRADE_TARGET_MINGW) && defined(CORRADE_TARGET_CLANG))
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wreturn-type-c-linkage"
 #endif