Simplify x2p's Makefile.

Simplify x2p's Makefile.

Eliminate plexe and plc, which relate to tests for the compiler and were
added as part of commit 52cebf5efc9883c7 in May 1988. The compiler was
removed by commit de1254415ffeb03b in Sept 2006.

The files a2p.man and s2p.man were removed with Perl 5.000.
The macros 'public', 'private' and 'util' mirror the structure of the top
level Makefile.SH. However, x2p only ever used 'public', so simplify things
by inlining the value of 'public' in the only place that uses it.
'addedbyconf' now only contains the value of 'plextract', so use the latter
directly in the one place that had used 'addedbyconf'.