COMP: do not include the (buggy BTW) internal liblas includes unconditionnally to support external liblas mode

COMP: do not add OSGEO4W_LIB in find_library PATHS. variable do not exist anymore and this is done in FindOSGeo4W

COMP: no need to add OSGEO4W_INCLUDE in find_path, this is done by FindOSGeo4W (actually the OSGEO4W_INCLUDE var is not even defined anymore)

COMP: use GDAL_CONFIG defined by FindGDAL.cmake instead of redefining a GDALCONFIG_EXECUTABLE variable