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

WRG: fix for variable 'prolateCoefficients' is not needed and will not be emitted [-Wunneeded-internal-declaration]

This should fix compilation on ArchLinux with gcc4.7/boost1.50
See also https://svn.boost.org/trac/boost/ticket/6940

See also fix sent upstream :
https://sourceforge.net/tracker/?func=detail&aid=3571921&group_id=240806&atid=1114094

This enables compilation on MacOSX 10.8, and (most probably) with Intel Compiler also