dispatch: introduce dispatch_STATIC and honour dispatch_EXPORT

When building with CMake, `dispatch_EXPORT` is automatically defined
when building a shared library. We should additionally rely on
`dispatch_STATIC` to indicate that we are building a static library.
When building statically, we should ensure that the ABI of dispatch does
not participate in dynamic linking as otherwise any consuming library
would vend the dispatch ABI. Particularly for Linux, this would be
problematic as the symbol resolution is ill-defined and can end up with
multiple incompatible definitions.

Author: compnerd

dispatch/base.h

@@ -186,16 +186,24 @@
 # endif
 #define
 
-#if defined(_WIN32)
-#if defined(__DISPATCH_BUILDING_DISPATCH__)
-#define DISPATCH_EXPORT DISPATCH_EXTERN __declspec(dllexport)
-#else
-#define DISPATCH_EXPORT DISPATCH_EXTERN __declspec(dllimport)
-#endif
-#elif __GNUC__
-#define DISPATCH_EXPORT DISPATCH_EXTERN __attribute__((visibility("default")))
+#if defined(dispatch_STATIC)
+# if defined(__GNUC__)
+#   define DISPATCH_EXPORT DISPATCH_EXTERN __attribute__((__visibility__("hidden")))
+# else
+#   define DISPATCH_EXPORT DISPATCH_EXTERN
+# endif
 #else
-#define DISPATCH_EXPORT DISPATCH_EXTERN
+# if defined(_WIN32)
+#   if defined(dispatch_EXPORT) || defined(__DISPATCH_BUILDING_DISPATCH__)
+#     define DISPATCH_EXPORT DISPATCH_EXTERN __declspec(dllexport)
+#   else
+#     define DISPATCH_EXPORT DISPATCH_EXTERN __declspec(dllimport)
+#   endif
+# elif __GNUC__
+#   define DISPATCH_EXPORT DISPATCH_EXTERN __attribute__((__visibility__("default")))
+# else
+#   define DISPATCH_EXPORT DISPATCH_EXTERN
+# endif
 #endif
 
 #if __GNUC__