stdckdint-h: Don't generate header if it is not needed.

Suggested by Paul Eggert in:
<https://lists.gnu.org/archive/html/bug-gnulib/2025-05/msg00216.html>.

* m4/stdckdint_h.m4: New file.
* modules/stdckdint-h (Files): Add m4/stdckdint_h.m4.
(configure.ac): Remove checks and just invoke gl_STDCKDINT_H.
(Makefile.am): Replace variables set by ./configure.
* lib/stdckdint.in.h: Include the compilers header if it exists.  Add
comment mentioning expected future standardization in C++26.
(ckd_add, ckd_sub, ckd_mul): Only define if the compilers definitions do
not work.

modules/stdckdint-h

@@ -2,6 +2,7 @@ Description:
 An <stdckdint.h> that is like C23.
 
 Files:
+m4/stdckdint_h.m4
 lib/stdckdint.in.h
 lib/intprops-internal.h
 
@@ -10,16 +11,7 @@ gen-header
 bool
 
 configure.ac:
-AC_CHECK_HEADERS_ONCE([stdckdint.h])
-if test $ac_cv_header_stdckdint_h = yes; then
-  if test -n "$CXX" && test "$CXX" != no; then
-    GL_GENERATE_STDCKDINT_H=true
-  else
-    GL_GENERATE_STDCKDINT_H=false
-  fi
-else
-  GL_GENERATE_STDCKDINT_H=true
-fi
+gl_STDCKDINT_H
 gl_CONDITIONAL_HEADER([stdckdint.h])
 AC_PROG_MKDIR_P
 
@@ -32,6 +24,15 @@ if GL_GENERATE_STDCKDINT_H
 stdckdint.h: stdckdint.in.h $(top_builddir)/config.status
 @NMD@	$(AM_V_GEN)$(MKDIR_P) '%reldir%'
 	$(gl_V_at)$(SED_HEADER_STDOUT) \
+	      -e 's|@''GUARD_PREFIX''@|${gl_include_guard_prefix}|g' \
+	      -e 's|@''INCLUDE_NEXT''@|$(INCLUDE_NEXT)|g' \
+	      -e 's|@''PRAGMA_SYSTEM_HEADER''@|@PRAGMA_SYSTEM_HEADER@|g' \
+	      -e 's|@''PRAGMA_COLUMNS''@|@PRAGMA_COLUMNS@|g' \
+	      -e 's|@''NEXT_STDCKDINT_H''@|$(NEXT_STDCKDINT_H)|g' \
+	      -e 's|@''HAVE_C_STDCKDINT_H''@|$(HAVE_C_STDCKDINT_H)|g' \
+	      -e 's|@''HAVE_WORKING_C_STDCKDINT_H''@|$(HAVE_WORKING_C_STDCKDINT_H)|g' \
+	      -e 's|@''HAVE_CXX_STDCKDINT_H''@|$(HAVE_CXX_STDCKDINT_H)|g' \
+	      -e 's|@''HAVE_WORKING_CXX_STDCKDINT_H''@|$(HAVE_WORKING_CXX_STDCKDINT_H)|g' \
 	  $(srcdir)/stdckdint.in.h > $@-t
 	$(AM_V_at)mv $@-t $@
 else