Switch NLS build to use the in-built ENABLE_NLS define rather than USE_NLS.

Switch NLS build to use the in-built ENABLE_NLS define rather than USE_NLS.

The key concept here is that we eliminate all logic from the Makefile and put
it in the build system (e.g. configure). By having the logic at this higher
layer, we make it much easier to transition to another build system in future
such as CMake.

git-svn-id: http://svn.osgeo.org/postgis/trunk@8808 b70326c6-7e19-0410-871a-916f4a2858ee