add support for long-opts (#332)

Uses [optparse] to add support for long-opts. optparse is posix
compliant with getopt(3) and thus would be backwards compatible.
It does not have any dependency (not even the c standard library!) and
is C89 compatible and thus fits our current code-style.

[optparse]: https://github.com/skeeto/optparse

Note that we're using a couple `pragma`-s to silence some harmless
warnings. This should be portable because these pragma-s don't change the
behavior of the program. Furthermore, C standard mandates that unknown
pragma's should be ignored by the compiler and thus would not result in
build failure on compilers which do not recognize them.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/328
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/332
Reviewed-by: eylles <eylles@noreply.codeberg.org>

Makefile

@@ -41,7 +41,7 @@ nsxiv: $(objs)
 	$(CC) $(CFLAGS) $(nsxiv_cppflags) -c -o $@ $<
 
 $(objs): Makefile config.mk nsxiv.h config.h commands.h
-options.o: version.h
+options.o: version.h optparse.h
 window.o: icon/data.h utf8.h
 
 config.h: