usrlocalpkg/admin/makebin: make /etc/mxpkg a variable

'/etc/mxpkg' might get disted over whilst testing things,
so provide an option for own experiments.

+ salute a vi :)

usrlocalpkg/admin/makebin

@@ -4,6 +4,7 @@ use warnings;
 
 # see /usr/local/package/admin/config for further descriptions
 
+my $MXPKG  = $ENV{MAKEBIN_MXPKG}  || '/etc/mxpkg';
 my $CFG    = $ENV{MAKEBIN_CFG}    || '/usr/local/package/admin/config';
 my $LIBSRC = $ENV{MAKEBIN_LIBSRC} || '/usr/local/package/admin/lib';
 my $BINDIR = $ENV{MAKEBIN_BINDIR} || '/usr/local/package/bin';
@@ -34,7 +35,7 @@ sub tilde_replace {
 
 sub read_mxpkg {
 	my @ret;
-	open my $in,'<',"/etc/mxpkg" or die "/etc/.xmpg: $!\n";
+	open my $in,'<',$MXPKG or die "$MXPKG: $!\n";
 	while(<$in>) {
 		s/#.*//;
 		/\S/ or next;