Use Mono prefix for libMonoPosixHelper

The libMonoPosixHelper is installed together with Mono, hence its path
in the configuration should be relative to the Mono prefix.

It was previously assumed to reside in a system path, so the the
dynamic linker would find it anyway.

This patch is based on the one included in bug #18555 (by Gaëtan
Lehmann <gaetan.lehmann@gmail.com>), but it also updates
runtime/Makefile.am to keep runtime/etc/mono/config consistent.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=18555
Improve libgdiplus path handling

Since libgdiplus is not installed together with Mono, it should not be
assumed to be in the same prefix.

The new policy for the --with-libgdiplus configure option is the
following:

 - --with-libgdiplus=/absolute/path/to/libgdiplus.{so,dylib}

   both during build/check and after installation Mono will try to use
   the specified libgdiplus library; the rationale is that when an
   absolute path is given, it can be assumed to be the full path to
   the library that is already installed (possibly in a non-default
   path).

 - default, --with-libgdiplus=no, --with-libgdiplus=installed

   both during build/check and after installation Mono will try to use
   a system-wide libgdiplus library, that is assumed to reside in the
   paths that are automatically searched by the dynamic linker; the
   library is supposed to be already installed in the default path and
   to be useable both during the build and afterwards.

 - --with-libgdiplus=sibling, --with-libgdiplus=yes

   during build/check Mono will try to use the libgdiplus library that
   is assumed to be in the sibling folder (../libgdiplus), but after
   the installation it will try to use a system-wide libgdiplus
   library, that should be in the paths that are automatically
   searched by the dynamic linker; the assumption is that the library
   is not yet installed, but it will go to the default prefix after
   installing it.

 - --with-libgdiplus=../some/relative/path/to/libgdiplus.{so,dylib,la}

   during build/check Mono will try to use the specified libgdiplus
   library, but after the installation it will try to use a
   system-wide libgdiplus library, that should be in the paths that
   are automatically searched by the dynamic linker; the assumption is
   that the library is not yet installed, but it will go to the
   default prefix after installing it.
Merge pull request #1064 from ranma42/fix-config-paths

Improve handling of lib paths for libMonoPosixHelper and libgdiplus. Fixes #18555.