Make all OGR drivers optional (but the Memory one)!

[rouault]

• Make OGR_VRT driver optional and buildable as plugin
• Make KML driver optional and buildable as plugin
• Make Shapefile driver optional and buildable as plugin (http://switchfromshapefile.org/ you're happy, aren't you ?). Note that if you disable the shapefile driver, the autotest suite will fail on a significant number of tests...
• Make GeoJSON (and companion TopoJSON, ESRIJSON, GeoJSONSeq) drivers optional or buildable as a plugin (if not builtin, the JSONFG driver is not available)
• Make MapInfo driver optional or buildable as a plugin (if not builtin, Shapefile attribute indices are not available)

A build with -DGDAL_BUILD_OPTIONAL_DRIVERS=OFF -DOGR_BUILD_OPTIONAL_DRIVERS=OFF now leads to:

$ ogrinfo --formats
Supported Formats: (ro:read-only, rw:read-write, +:update, v:virtual-I/O s:subdatasets)
  Memory -vector- (rw+): Memory