xbps-src: added XBPS_ALLOW_RESTRICTED conf option for restricted pkgs.

Such packages should set the `restricted' var to allow building a binary package.
Note that such packages do not allow redistribution of sources and binaries,
so that it's up to the user if (s)he wants to pkg it locally.

etc/defaults.conf

@@ -88,6 +88,12 @@ XBPS_SUCMD="sudo /bin/sh -c"
 #
 #XBPS_PKG_OPTIONS_foo=opt,~opt2,opt3,~opt4
 
+# [OPTIONAL]
+# Enable building package locally that are restricted legally for redistribution.
+# NOTE: you can't distribute the sources or binaries for such kind of packages.
+#
+#XBPS_ALLOW_RESTRICTED=yes
+
 # [OPTIONAL]
 # Set the preferred chroot style. Available styles at common/chroot-style/*.sh:
 #