package/Makefile.in: fix musl handling

Until now, we had no support for full NLS with the musl C library:
BR2_NEEDS_GETTEXT was only true for uClibc. But the musl C library
provides a stub gettext implementation, which some packages were
failing to recognize as being usable, and therefore we are passing
autoconf cache variables to hint those packages that yes, the C
library has a usable gettext implementation.

However, we are going to enable full NLS support for musl, by giving
the possibility to build gettext libintl with musl. In such a case, we
do not want packages to use the gettext implementation of the C
library, but really the one provided by gettext libintl.

Therefore, we should only pre-seed the
gt_cv_func_gnugettext1_libc*=yes variables if we're on musl but
without gettext libintl. Otherwise packages will fail building because:

 - libintl.h is the one from the full-blown gettext implementation, so
   it assumes the package will link against -lintl

 - the package thinks gettext is provided by the C library, so it
   doesn't link with -lintl

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>