ExtUtils::MM_Unix: Explicitly link against libperl.so on shared perls

Under Android, modules won't recognize symbols from libperl unless
it's explicitly declared as a dependency when linked.
MM_Unix: On Android, pass PERL_LIB through rel2abs().

Some unusual behavior, probably in Android's linker, makes these work:

    $ mkdir -p one/two/three
    $ ./perl -Ilib -MFile::Glob -e1
    $ cd one
    $ ../perl -I../lib -MFile::Glob -e1
    $ cd two
    $ ../../perl -I../../lib -MFile::Glob -e1

But this fails:

    $ cd three
    $ ../../../perl -I../../../lib -MFile::Glob -e1
    Can't load '../../../lib/auto/File/Glob/PL_File__Glob.so' for module File::Glob: Cannot load library: load_library[1093]: Library '../../../lib/auto/File/Glob/PL_File__Glob.so' not found at ../../../lib/DynaLoader.pm line 212.
     at -e line 0.
    Compilation failed in require.
    BEGIN failed--compilation aborted.

This is a problem for the core tests, and possibly for perls built with
-Duserelocatableinc.

We work around it by passing PERL_LIB through rel2abs().