maint: be more robust to gnulib's FOO_H variables.

* configure.ac: Instead of listing gnulib's variables, look for
	them among AC_SUBST variables.

configure.ac

@@ -168,11 +168,15 @@ gt_JAVAEXEC
 # We use gnulib, but from lib/local.mk instead of lib/Makefile.am.
 # So prefix paths with lib/.  See also etc/prefix-gnulib-mk.
 gl_LIBOBJS=`echo "$gl_LIBOBJS" | sed -e 's, , lib/,g'`
-for ac_var in ALLOCA_H ERRNO_H FCNTL_H GETOPT_H INTTYPES_H \
-              LIBUNISTRING_UNITYPES_H LIBUNISTRING_UNIWIDTH_H \
-              STDBOOL_H STDINT_H SYS_WAIT_H WCHAR_H WCTYPE_H
+# Listing the names of the variables to prefix is error-prone.
+# Rather, adjust all the AC_SUBST variables whose name ends in '_H',
+# and whose content ends in '.h'.
+for ac_var in $ac_subst_vars
 do
-  eval "test x\$$ac_var = x || $ac_var=lib/\$$ac_var"
+  eval "ac_val=\$$ac_var"
+  case $ac_var=$ac_val in
+    (*_H=*.h) eval "$ac_var=lib/\$$ac_var";;
+  esac
 done
 AC_CONFIG_FILES([Makefile
                  po/Makefile.in