added CMake option GMX_PREFER_STATIC_LIBS (also fixes bug #541)

added CMake option GMX_PREFER_STATIC_LIBS (also fixes bug #541)

This option instructs CMake to prefer static external libs when both static
and shared version available. This makes possible linking statically against
external libraries (e.g. fftw, xml2).

The option only available on *NIX platforms, except APPLE and CYGWIN.

Note, that when the option is enabled UILD_SHARED_LIBS gets disabled.
Also note, that with this option on, the binaries built will still be
dynamically linked against system libraries.