Now configure properly takes into account paths such as
--bindir=/path
etc

This implementation hardcodes all the path into the
src/config/ConfigInstall.cpp file. I followed the instructions on autoconf
documentation so that the paths can be changed at install time using e.g.

make install prefix=/path bindir=/path/binxx

This should be the standard behavior.

I removed the possibility to use PLUMED_SUFFIX and PLUMED_LIBPREFIX
since it was a bit too difficult to maintain.

I significantly simplified the src/lib/Makefile by using
the standard directory names for autoconf.