Use -Wtrailing-whitespace when warning

* configure.ac: When enabling GCC warnings, enable GCC 15’s new
-Wtrailing-whitespace option if available, as that’s the Emacs style.

configure.ac

@@ -1851,6 +1851,7 @@ AS_IF([test $gl_gcc_warnings = no],
     gl_WARN_ADD([$w])
   done
   gl_WARN_ADD([-Wredundant-decls])     # Prefer this, as we don't use Bison.
+  gl_WARN_ADD([-Wtrailing-whitespace]) # This project's coding style
   gl_WARN_ADD([-Wno-missing-field-initializers]) # We need this one
   gl_WARN_ADD([-Wno-override-init])    # More trouble than it is worth
   gl_WARN_ADD([-Wno-sign-compare])     # Too many warnings for now