GL: expand the Clang-CL warning suppression to regular Clang as well.

Both are a thing on Windows, so both need the same.

src/Magnum/GL/Context.cpp

@@ -673,9 +673,10 @@ 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 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_CL
+/* 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
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wreturn-type-c-linkage"
 #endif
@@ -683,7 +684,7 @@ extern "C" CORRADE_VISIBILITY_EXPORT Context*& magnumGLUniqueCurrentContext();
 extern "C" CORRADE_VISIBILITY_EXPORT Context*& magnumGLUniqueCurrentContext() {
     return currentContext;
 }
-#ifdef CORRADE_TARGET_CLANG_CL
+#ifdef CORRADE_TARGET_CLANG
 #pragma clang diagnostic pop
 #endif