Allow precompiled headers to be enabled from command-line
Use find modules for libraries with extra processing

Some libraries require more information than what is present in the
xxx-config.cmake file, e.g. the caller must know whether HAVE_TUPLE
is available and probably used when compiling dune-common, and put
this in its own config.h file.

Code to take care of these variables must therefore be in the client
configuration, and this is the same code which is used to handle the
autotools version, namely the find module, so a practical solution
is to just revert to that in both cases.
Don't search for source in build directories
Search for libraries in dunecontrol's default build dir
Merge pull request #204 from rolk/204_precomp

Allow precompiled headers to be enabled from command-line