Handle LIBDISCID_API for mingw and cygwin

Also move config.h include to the top of disc.c so that the functions are built
are marked as dllexport while building the library.
Use winver.h instead of Winver.h
Honor visibility in src/base64.c

Signed-off-by: Sebastian Ramacher <sramacher@debian.org>
Merge branch 'hide-private-symbols'

Conflicts:
	include/discid/discid.h