Update the symbol visibility rules under MSVC

For the Visual Studio C compiler we need to annotate our public symbols
with `__declspec(dllimport)` to ensure they are visible when dynamically
linking to Epoxy's DLL.

This is needed because under Windows we use a dispatch table, instead of
wrapper functions, thus the symbol visibility rules change. Compiling
with MingW will automatically add `__declspec(dllimport)` for us.

Thanks to Nirbheek Chauhan for the help in debugging the issue.

Fixes #104

include/epoxy/common.h

@@ -38,7 +38,11 @@
 #endif
 
 #ifndef EPOXY_PUBLIC
-# define EPOXY_PUBLIC extern
+# if defined(_MSC_VER)
+#  define EPOXY_PUBLIC __declspec(dllimport) extern
+# else
+#  define EPOXY_PUBLIC extern
+# endif
 #endif
 
 #if defined(_MSC_VER) && !defined(__bool_true_false_are_defined) && (_MSC_VER < 1800)