Export globals and make them weak on static builds so they work across SOs.

Well, except on Windows -- not yet. Added an XFAIL for that.

doc/changelog.dox

@@ -293,6 +293,10 @@ See also:
 -   @ref Image::data(), @ref CompressedImage::data() and
     @ref Trade::ImageData::data() called on a r-value now return
     @ref Corrade::Containers::Array instead of being deleted
+-   Global state used by @ref GL::Context and @ref Audio::Context is no longer
+    duplicated when Magnum is built statically and linked to more than one
+    dynamic library or executable. Currently that holds only for Linux and
+    macOS, Windows support will come next.
 
 @subsubsection changelog-latest-changes-animation Animation library
 

src/Magnum/Audio/Context.cpp

@@ -113,11 +113,31 @@ std::vector<std::string> Context::deviceSpecifierStrings() {
     return list;
 }
 
+#ifndef MAGNUM_BUILD_STATIC
+/* (Of course) can't be in an unnamed namespace in order to export it below */
 namespace {
-    /* Unlike GL, this isn't thread-local. Would need to implement
-       ALC_EXT_thread_local_context first */
-    Context* currentContext = nullptr;
+#endif
+
+/* Unlike GL, this isn't thread-local. Would need to implement
+   ALC_EXT_thread_local_context first */
+#if defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS)
+/* On static builds that get linked to multiple shared libraries and then used
+   in a single app we want to ensure there's just one global symbol. On Linux
+   it's apparently enough to just export, macOS needs the weak attribute.
+   Windows not handled yet, as it needs a workaround using DllMain() and
+   GetProcAddress(). */
+CORRADE_VISIBILITY_EXPORT
+    #ifdef __GNUC__
+    __attribute__((weak))
+    #else
+    /* uh oh? the test will fail, probably */
+    #endif
+#endif
+Context* currentContext = nullptr;
+
+#ifndef MAGNUM_BUILD_STATIC
 }
+#endif
 
 bool Context::hasCurrent() { return currentContext; }
 

src/Magnum/Audio/Test/GlobalStateAcrossLibrariesALTest.cpp

@@ -44,7 +44,12 @@ GlobalStateAcrossLibrariesALTest::GlobalStateAcrossLibrariesALTest() {
 void GlobalStateAcrossLibrariesALTest::test() {
     Context context;
     CORRADE_VERIFY(Context::hasCurrent());
-    CORRADE_COMPARE(currentContextInALibrary(), &context);
+    {
+        #ifdef CORRADE_TARGET_WINDOWS
+        CORRADE_EXPECT_FAIL("Deduplication of global data across shared libraries isn't implemented on Windows yet.");
+        #endif
+        CORRADE_COMPARE(currentContextInALibrary(), &context);
+    }
 }
 
 }}}}

src/Magnum/GL/Context.cpp

@@ -466,12 +466,32 @@ Containers::ArrayView<const Extension> Extension::extensions(Version version) {
     CORRADE_ASSERT_UNREACHABLE(); /* LCOV_EXCL_LINE */
 }
 
+#ifndef MAGNUM_BUILD_STATIC
+/* (Of course) can't be in an unnamed namespace in order to export it below */
 namespace {
-    #ifdef CORRADE_BUILD_MULTITHREADED
-    CORRADE_THREAD_LOCAL
+#endif
+
+#ifdef CORRADE_BUILD_MULTITHREADED
+CORRADE_THREAD_LOCAL
+#endif
+#if defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS)
+/* On static builds that get linked to multiple shared libraries and then used
+   in a single app we want to ensure there's just one global symbol. On Linux
+   it's apparently enough to just export, macOS needs the weak attribute.
+   Windows not handled yet, as it needs a workaround using DllMain() and
+   GetProcAddress(). */
+CORRADE_VISIBILITY_EXPORT
+    #ifdef __GNUC__
+    __attribute__((weak))
+    #else
+    /* uh oh? the test will fail, probably */
     #endif
-    Context* currentContext = nullptr;
+#endif
+Context* currentContext = nullptr;
+
+#ifndef MAGNUM_BUILD_STATIC
 }
+#endif
 
 bool Context::hasCurrent() { return currentContext; }
 

src/Magnum/GL/Test/GlobalStateAcrossLibrariesGLTest.cpp

@@ -42,7 +42,12 @@ GlobalStateAcrossLibrariesGLTest::GlobalStateAcrossLibrariesGLTest() {
 
 void GlobalStateAcrossLibrariesGLTest::test() {
     CORRADE_VERIFY(GL::Context::hasCurrent());
-    CORRADE_COMPARE(currentContextInALibrary(), &GL::Context::current());
+    {
+        #ifdef CORRADE_TARGET_WINDOWS
+        CORRADE_EXPECT_FAIL("Deduplication of global data across shared libraries isn't implemented on Windows yet.");
+        #endif
+        CORRADE_COMPARE(currentContextInALibrary(), &GL::Context::current());
+    }
 }
 
 }}}}