freeswitch-stable: backport iconv fixes

freeswitch-stable: backport iconv fixes

Backport iconv fixes from master.

- Include nls.mk and remove the uclibc iconv hack.

- The above change pointed to an issue with the unimrcp sed script. It
  is missing brackets, which causes follow-up breakage when APR_SETVAR
  is called, because the latter uses commas to separate its arguments -
  commas that TARGET_LDFLAGS contains when nls.mk is included. So add
  the missing brackets.

- mod_gsmopen is unable to detect whether iconv's inbuf is const. Update
  the patch so the Makefile can pass this information on to mod_gsmopen.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>