SunOS getlocalename_l() redeclaration

[outpaddling]

Building devel/m4 and several gettext-* packages on Openindiana errors out with the following:

  CC       localename.o
localename.c:55:15: error: conflicting types for 'getlocalename_l'; have 'char *(int,  struct _locale *)'
   55 | extern char * getlocalename_l(int, locale_t);
      |               ^~~~~~~~~~~~~~~
In file included from ./locale.h:41,
                 from localename.c:34:
/usr/include/locale.h:98:20: note: previous declaration of 'getlocalename_l' with type 'const char *(int,  struct _locale *)'
   98 | extern const char *getlocalename_l(int, locale_t);
      |                    ^~~~~~~~~~~~~~~
*** [localename.o] Error code 1

bmake[3]: stopped making "all-am" in /export/home/bacon/Dreckly/dreckly/devel/m4/work/m4-1.4.19/lib

It's easily worked around by commenting out the hacked-in prototype in localename.c, but I'm concerned that doing this will break builds on other SunOS-based systems, to which I have no direct access for testing.

Can someone working with SunOS derivatives could propose a safe solution?