Audio, GL: (attempt to) fix MinGW GCC warnings on static Windows builds.

The CI doesn't have this configuration in the build matrix, so hoping
it's doing what it's supposed to.

src/Magnum/Audio/Context.cpp

@@ -142,8 +142,12 @@ CORRADE_VISIBILITY_EXPORT
 Context* currentContext = nullptr;
 #else
 /* On Windows the symbol is exported unmangled and then fetched via
-   GetProcAddress() to emulate weak linking. */
-extern "C" CORRADE_VISIBILITY_EXPORT Context* magnumAudioUniqueCurrentContext = nullptr;
+   GetProcAddress() to emulate weak linking. Using an extern "C" block instead
+   of just a function annotation because otherwise MinGW prints a warning:
+   '...' initialized and declared 'extern' (uh?) */
+extern "C" {
+    CORRADE_VISIBILITY_EXPORT Context* magnumAudioUniqueCurrentContext = nullptr;
+}
 #endif
 
 #if !defined(MAGNUM_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)

src/Magnum/GL/Context.cpp

@@ -513,6 +513,7 @@ 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) && !defined(CORRADE_TARGET_WINDOWS_RT)
+extern "C" CORRADE_VISIBILITY_EXPORT Context*& magnumGLUniqueCurrentContext();
 extern "C" CORRADE_VISIBILITY_EXPORT Context*& magnumGLUniqueCurrentContext() {
     return currentContext;
 }