* unlink-busy.m4 (gl_FUNC_UNLINK_BUSY_TEXT): Use prototype to define main with arguments, for C++. Reported by Eric Blake. * c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Prefer 'int main ()' to 'int main (void)', for C++. * getcwd-path-max.m4 (gl_FUNC_GETCWD_PATH_MAX): Likewise. * fsusage.m4 (gl_FILE_SYSTEM_USAGE): Specify a return type for 'main', for C99 and C++.