Build failure with stricter C compilers (e.g. GCC 14)

Modern C compilers are becoming stricter with a variety of changes over the last year or so.

GCC 14 in particular (to be released in ~April 2024) fails to build xkbutils-1.0.5 like:

xkbwatch.c:92:34: error: passing argument 7 of ‘XtOpenApplication’ from incompatible pointer type [-Wincompatible-pointer-types]
   92 |                                  fallback_resources,
      |                                  ^~~~~~~~~~~~~~~~~~
      |                                  |
      |                                  char **
In file included from xkbwatch.c:31:
/usr/include/X11/Intrinsic.h:1473:5: note: expected ‘const char **’ but argument is of type ‘char **’
 1473 |     String*             /* fallback_resources */,
      |     ^~~~~~~                   ^

Originally reported downstream in Gentoo at https://bugs.gentoo.org/919190.

This can be emulated with -Werror=incompatible-pointer-types -Werror=implicit -Werror=int-conversion on an older GCC or Clang.