madplay: fix musl gettext support detection

Fixes:
http://autobuild.buildroot.net/results/256/2561190b274d71666c4bdf3c569b02063cefdb30/
http://autobuild.buildroot.net/results/14e/14e6addcd3ec35f882da7ec489caa9b60ecd4b63/
http://autobuild.buildroot.net/results/cf0/cf011286be839674d358a8bccaf1c5c52de75e46/

madplay needs gettext when built with nls, and it support 3 variants:

- C library support gettext (E.G. glibc)
- Libintl (E.G. uClibc)
- An included libintl copy

The included libintl copy has unfortunately bitrotted and doesn't even build
any more.  With that said, musl DOES have gettext support, so that should be
used instead.

The configure script unfortunately uses an old AM_GNU_GETTEXT test, which
explicitly checks for nonstandard glibc extensions, which musl doesn't
support:

http://www.openwall.com/lists/musl/2015/04/16/3



Which causes the detection to fail:

configure:24770: checking for GNU gettext in libc
configure:24794: /home/peko/source/buildroot/output/host/usr/bin/i486-linux-musl-gcc \
   -o conftest -Wall -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 \
   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  conftest.c  >&5
/tmp/ccboDFhK.o: In function `main':
conftest.c:(.text+0x39): undefined reference to `_nl_msg_cat_cntr'
conftest.c:(.text+0x40): undefined reference to `_nl_domain_bindings'
collect2: error: ld returned 1 exit status

Now, madplay itself doesn't actually use these glibc extensions, so just force the
detection of GNU gettext when musl is used.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>