Fix --enable-universalsdk and its comment line so that zsh's flag completion
works.

Thanks to Jeroen Ruigrok van der Werven for the fix.
diff --git a/configure.in b/configure.in
index f421eab..bd16b2e 100644
--- a/configure.in
+++ b/configure.in
@@ -61,7 +61,7 @@
 CONFIG_ARGS="$ac_configure_args"
 
 AC_ARG_ENABLE(universalsdk,
-	AC_HELP_STRING(--enable-universalsdk@<:@SDKDIR@:>@, Build against Mac OS X 10.4u SDK (ppc/i386)),
+	AC_HELP_STRING(--enable-universalsdk@<:@=SDKDIR@:>@, Build against Mac OS X 10.4u SDK (ppc/i386)),
 [
 	case $enableval in
 	yes)