Have the generated .c code decorate the prototypes with "G_MODULE_EXPORT" instead of "extern" when --internal is not being used, so that we also export the symbols from the generated code on Visual Studio-style compilers. If --internal is used, we decorate the prototypes with "G_GNUC_INTERNAL", as we did before.
Note that since the generated .c code does not attempt to include the generated headers (if one is also generated), the gnerated headers are still generated as they were before.
Trivial backport of !1468 (merged) to glib-2-64.