package/asterisk: enable for uclibc toolchains

package/asterisk: enable for uclibc toolchains

This patch extends the configure checks for re-entrant resolver
functions to fix uclibc builds.

Quoting Yann:
http://lists.busybox.net/pipermail/buildroot/2017-September/203004.html

"As a final stroke of genius, asterisk checks for the re-entrant variant
of res_ninit(), and concludes that all such functions are available,
including res_nsearch(). Uclibc-ng has the former but not the latter, so
the build fails. Since there is no cache variable for that check, we
can't pre-feed that result to configure, and fixing it is a bigger
endeavour. So we make asterisk depend on glibc for now, until someone
is brave enough to fix it."

Musl builds are still broken:

output/build/asterisk-16.0.0/include/asterisk/astmm.h:165:35:
 error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘->’ token
  Do_not_use_calloc__use_ast_calloc->fail(a, b)

output/build/asterisk-16.0.0/include/asterisk/astmm.h:169:77:
 error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘->’ token
  Do_not_use_free__use_ast_free_or_ast_std_free_for_remotely_allocated_memory->fail(a)

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>