config-scripts: Fix --enable-libtool-unsupported

When the LIBTOOL variable is set in the user's environment the build
will use libtool even when --enable-libtool-unsupported is not used.

This can lead to unexpected failures when LIBTOOL is set to rlibtool
rather than GNU libtool.

To solve this the build now relies on the enable_libtool_unsupported
varaiable set by --enable-libtool-unsupported.

Gentoo bug: https://bugs.gentoo.org/show_bug.cgi?id=843638

Author: orbea

config-scripts/cups-libtool.m4

@@ -9,20 +9,21 @@ dnl Licensed under Apache License v2.0.  See the file "LICENSE" for more
 dnl information.
 dnl
 
-AC_ARG_ENABLE([libtool_unsupported], AS_HELP_STRING([--enable-libtool-unsupported=/path/to/libtool], [build with libtool (UNSUPPORTED)]), [
-    AS_IF([test x$enable_libtool_unsupported != xno], [
-	AS_IF([test x$enable_libtool_unsupported = xyes], [
-	    AC_MSG_ERROR([Use --enable-libtool-unsupported=/path/to/libtool.])
-	])
-	LIBTOOL="$enable_libtool_unsupported"
-	enable_shared="no"
-	AC_MSG_WARN([WARNING: libtool is not supported.])
-    ], [
-	LIBTOOL=""
+AC_ARG_ENABLE([libtool_unsupported],
+    AS_HELP_STRING([--enable-libtool-unsupported=/path/to/libtool],
+        [build with libtool (UNSUPPORTED)]),
+    [], [enable_libtool_unsupported=no])
+
+AS_IF([test x$enable_libtool_unsupported != xno], [
+    AS_IF([test x$enable_libtool_unsupported = xyes], [
+        AC_MSG_ERROR([Use --enable-libtool-unsupported=/path/to/libtool.])
+    ],[
+        AC_MSG_WARN([libtool is not supported.])
     ])
-])
 
-AS_IF([test x$LIBTOOL != x], [
+    LIBTOOL="$enable_libtool_unsupported"
+    enable_shared="no"
+
     DSO="\$(LIBTOOL) --mode=link --tag=CC ${CC}"
     DSOXX="\$(LIBTOOL) --mode=link --tag=CXX ${CXX}"