First draft of symbol visibility macros

For GCC 4+ these now produce a nice clean build when compiling the split
compiler source files.  Without these, the result is quite messy.

MSVC symbol visibility macros are based on feedback from David Demelier.
They seem to now allow a Windows DLL build (__declspec(dllimport) and
__declspec(dllexport)).

src/duk_features.h.in

@@ -1897,6 +1897,98 @@ typedef FILE duk_file;
 #define DUK_UNLIKELY(x)  (x)
 #endif
 
+/*
+ *  Symbol visibility macros
+ */
+
+/* XXX: add support for at least clang */
+/* XXX: user override for these? user override for just using the default visibility macros? */
+
+#if defined(DUK_F_GCC_VERSION)
+#if (DUK_F_GCC_VERSION >= 40000)
+#define DUK_F_GCC_SYMBOL_VISIBILITY
+#endif
+#endif
+#if defined(_WIN32) && (defined(_MSC_VER) || defined(__GNUC__))
+/* MSVC, also MinGW on Windows. */
+#define DUK_F_MSVC_SYMBOL_VISIBILITY
+#endif
+
+/* FIXME: separate macros for functions and data may be necessary. */
+
+#if defined(DUK_F_GCC_SYMBOL_VISIBILITY)
+/* GCC 4+ visibility */
+#if defined(DUK_SINGLE_FILE)
+#define DUK_EXTERNAL_DECL  __attribute__ ((visibility("default"))) extern
+#define DUK_EXTERNAL       __attribute__ ((visibility("default")))
+#define DUK_INTERNAL_DECL  static
+#define DUK_INTERNAL       static
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#else  /* DUK_SINGLE_FILE */
+#define DUK_EXTERNAL_DECL  __attribute__ ((visibility("default"))) extern
+#define DUK_EXTERNAL       __attribute__ ((visibility("default")))
+#define DUK_INTERNAL_DECL  __attribute__ ((visibility("hidden"))) extern
+#define DUK_INTERNAL       __attribute__ ((visibility("hidden")))
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#endif
+#elif defined(DUK_F_MSVC_SYMBOL_VISIBILITY)
+/* MSVC dllexport/dllimport: appropriate __declspec depends on whether we're
+ * compiling Duktape or the application.
+ */
+#if defined(DUK_SINGLE_FILE)
+#if defined(DUK_COMPILING_DUKTAPE)
+#define DUK_EXTERNAL_DECL  extern __declspec(dllexport)
+#define DUK_EXTERNAL       __declspec(dllexport)
+#define DUK_INTERNAL_DECL  static
+#define DUK_INTERNAL       static
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#else
+#define DUK_EXTERNAL_DECL  extern __declspec(dllimport)
+#define DUK_EXTERNAL       should_not_happen
+#define DUK_INTERNAL_DECL  static
+#define DUK_INTERNAL       static
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#endif  /* DUK_COMPILING_DUKTAPE */
+#else  /* DUK_SINGLE_FILE */
+#if defined(DUK_COMPILING_DUKTAPE)
+#define DUK_EXTERNAL_DECL  extern __declspec(dllexport)
+#define DUK_EXTERNAL       __declspec(dllexport)
+#define DUK_INTERNAL_DECL  extern
+#define DUK_INTERNAL       /*empty*/
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#else
+#define DUK_EXTERNAL_DECL  extern __declspec(dllexport)
+#define DUK_EXTERNAL       should_not_happen
+#define DUK_INTERNAL_DECL  extern
+#define DUK_INTERNAL       /*empty*/
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#endif  /* DUK_COMPILING_DUKTAPE */
+#endif  /* DUK_SINGLE_FILE */
+#else
+/* Default visibility */
+#if defined(DUK_SINGLE_FILE)
+#define DUK_EXTERNAL_DECL  extern
+#define DUK_EXTERNAL       /*empty*/
+#define DUK_INTERNAL_DECL  static
+#define DUK_INTERNAL       static
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#else  /* DUK_SINGLE_FILE */
+#define DUK_EXTERNAL_DECL  extern
+#define DUK_EXTERNAL       /*empty*/
+#define DUK_INTERNAL_DECL  extern
+#define DUK_INTERNAL       /*empty*/
+#define DUK_LOCAL_DECL     static
+#define DUK_LOCAL          static
+#endif
+#endif
+
 /*
  *  __FILE__, __LINE__, __func__ are wrapped.  Especially __func__ is a
  *  problem because it is not available even in some compilers which try