Reported by Paul Eggert in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-01/msg00079.html>.
* m4/00gnulib.m4 (gl_COMPILER_CLANG, gl_COMPILER_PREPARE_CHECK_DECL):
Don't AC_REQUIRE anything.
(gl_COMPILER_PREPARE_CHECK_DECL): Define through AC_DEFUN, not
AC_DEFUN_ONCE. Use _AC_COMPILE_IFELSE, not AC_COMPILE_IFELSE.
(_AC_CHECK_DECL_BODY): If ac_compile_for_check_decl has not been set,
use ac_compile instead.
(AC_CHECK_DECL): Remove override.
* m4/~~gnulib.m4: New file.
* gnulib-tool (func_get_filelist): Add also ~~gnulib.m4.
* pygnulib/GLModuleSystem.py (getFiles): Likewise.