Add another dir to @*INC

In the long run I want panda and R* to install the modules there,
instead of ~./perl6/lib, which is shared among different Rakudo versions.
Currently this means that if you have precompiled modules in ~/.perl6/lib,
R* fails to compile modules.

You can find this new dir with perl6 -e 'say $*CUSTOM-LIB'
Better names are very welcome :-)

src/core/terms.pm

@@ -71,6 +71,9 @@ sub term:<time>() { nqp::p6box_i(nqp::time_i()) }
         @INC.push((%ENV<HOME> // %ENV<HOMEDRIVE> ~ %ENV<HOMEPATH>) ~ '/.perl6/lib');
     }
     @INC.push($VM<config><libdir> ~ $VM<config><versiondir> ~ '/languages/perl6/lib');
+    my $CUSTOM-LIB = $VM<config><libdir> ~ $VM<config><versiondir> ~ '/languages/perl6/custom-lib';
+    @INC.push($CUSTOM-LIB);
+    nqp::bindkey(pir::get_who__PP(PROCESS), '$CUSTOM-LIB', $CUSTOM-LIB);
 
     my $I := nqp::atkey(nqp::atkey(%*COMPILING, '%?OPTIONS'), 'I');
     if nqp::defined($I) {