Various fixes to cmake configuration

(1) Allow OSGEO4W_ROOT environment variable to override c:/OSGeo4W.

(2) Search for GDAL, GeoTIFF, and TIFF via the config method before
trying the older module method.

(3) Improve the way RPATH is set under Linux (use
CMAKE_INSTALL_RPATH_USE_LINK_PATH instead of setting the INSTALL_RPATH
property on the executables).

(4) On Windows, install the dependent DLLs in the bin directory.

(5) On Linux, install liblas-config.cmake in a directory without the
version number.  (The use of the version number could result in the
wrong library being found.)