* Bump GEOS version wherever necessary
* Update download package and checksum.
* Replace download from SVN with official release source package.
* Remove generating of geos_svn_revision.h as unnecessary -
released source package includes the header.
* Explain status of the missing CMake modules.
* Remove CMake option -DBUILD_TESTING as unused.
GEOS is important dependency of GDAL (updated in #1879;
GDAL 2.2.2 can still work with GEOS earlier than 3.6.2).
