Clean up compile flags

* packages/packages.common (scute, gpgol): Update for
most recent find scripts.
* src/Makefile.am: Remove obsolete config flags and
comments.
* src/gpg4win.mk.in: Set PKG_CONFIG_LIBDIR on ex builds, too.
Set --libdir on ex builds explicitly.

This should again make the code more easier to read.
The --with-gpg-error-prefix,.. flags etc. were mostly
required because PKG_CONFIG was not set up correctly.
In my case a PKG_CONFIG_LIBDIR set in the outside
environment leaked into the Gpg4win build. The
fewer custom flags we have, the better.