Revert guile configure to use guile-config

Replace pkg-config with guile-config to look for guile. Using pkg-config
requires the pkg.m4 autoconf macros to be properly installed on the
machine running autogen.sh which is frequently a problem. pkg-config
only supports more recent releases of guile so isn't very good at
finding guile either.
Portable dynamic library loading for Guile

dynamic-link and load-extension work without passing the .so or .dll as
the shared library extension, so these have been dropped so the examples
and test-suite work on Cygwin.

Also update documentation and use the 'lib' prefix as that is what we
commonly name the shared libraries.
Correct Guile Makefile to fix test-suite/examples on Cygwin
Fix running of Guile multivalue and std_vector examples
Tidy up target language versions display