gettext: don't add -lintl on musl, either

gettext won't actually provide libintl unless we go out of
our way to tell it to do so [1][2].

We could add those flags on musl (as I initially did in [3]),
but then we have two different libintl.h files and generally
some confusion about which gettext is being used.

Instead of sorting that out, for now let's just continue on
without gettext providing libintl-- it's worked well enough so far.

Only change that needs to be made, then, is to avoid
adding -lintl on musl since there is no libintl.

[1] https://github.com/pullmoll/void-packages/commit/c739240fd25e8378fe0467cd51ab483427337410
[2] https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-libs/libintl/libintl-0.19.8.1.ebuild?id=332e48712b6521697f992f923c9c985482dd1c36#n41
[3] https://github.com/dtzWill/nixpkgs/commit/729302f29aea8d380196d114e7d2f904add40711