build: Assume ANSI C declaration of 'setlocale'.
* gettext-tools/tests/setlocale.c (setlocale): Assume SETLOCALE_CONST is
'const'.
* gettext-tools/tests/format-c-3-prg.c: Likewise.
* gettext-tools/tests/format-c-4-prg.c: Likewise.
* gettext-tools/tests/plural-1-prg.c: Likewise.
* gettext-tools/tests/tstgettext.c: Likewise.
* gettext-tools/tests/tstngettext.c: Likewise.
* gettext-tools/configure.ac: Don't invoke gt_SETLOCALE.
* gettext-tools/m4/setlocale.m4: Remove file.
* gettext-tools/m4/Makefile.am (EXTRA_DIST): Remove it.
