build failure on NetBSD 9.0

Hi,

On NetBSD 9.0/x86_64, the newest release m4-1.4.18 exhibits a "make check"
failure:

  CC       test-localename.o
../../tests/test-localename.c: In function 'test_locale_name':
../../tests/test-localename.c:74:3: warning: implicit declaration of function 'uselocale'; did you mean 'setlocale'? [-Wimplicit-function-declaration]
   uselocale (LC_GLOBAL_LOCALE);
   ^~~~~~~~~
   setlocale
  CCLD     test-localename
ld: test-localename.o: in function `test_locale_name_thread':
/home/bruno/m4-1.4.18/build/tests/../../tests/test-localename.c:238: undefined reference to `uselocale'
ld: /home/bruno/m4-1.4.18/build/tests/../../tests/test-localename.c:249: undefined reference to `uselocale'
ld: /home/bruno/m4-1.4.18/build/tests/../../tests/test-localename.c:254: undefined reference to `uselocale'
ld: /home/bruno/m4-1.4.18/build/tests/../../tests/test-localename.c:277: undefined reference to `uselocale'
ld: /home/bruno/m4-1.4.18/build/tests/../../tests/test-localename.c:288: undefined reference to `uselocale'
ld: test-localename.o:/home/bruno/m4-1.4.18/build/tests/../../tests/test-localename.c:422: more undefined references to `uselocale' follow
*** Error code 1

This is fixed in the current m4 snapshot that can be downloaded from
https://gitlab.com/gnu-m4/ci-distcheck/

Bruno