configure: Fix gperf length parameter determination

gperf is not actually a build dependency as the generated files are
shipped in the tarball.  So the type depends on the gperf version on
the host that ran gperf and created the tarball, which might not be
the same as that on the actual build host, and gperf might not even
be installed there, leaving the type undetermined.

Fixes: e0e4322973 ("configure: Detect type of length parameter for gperf generated function")

configure.ac

@@ -392,7 +392,7 @@ if test -x "$GPERF"; then
 				[AC_MSG_ERROR([unable to determine gperf len type])]
 			)]
 		)
-		AC_DEFINE_UNQUOTED([GPERF_LEN_TYPE], [$GPERF_LEN_TYPE], [gperf len type])
+		AC_SUBST(GPERF_LEN_TYPE)
 		AC_MSG_RESULT([yes])
 	else
 		AC_MSG_RESULT([no])