BUILD: makefile: refactor support for 51DEGREES v3/v4

BUILD: makefile: refactor support for 51DEGREES v3/v4

In order to simplify maintenance and long-term evolutions, now the
feature remains enabled by setting USE_51DEGREES=1 and the version
is set in 51DEGREES_VER (3 or 4 are supported only). The default
version remains 3. All 51DEGREES flags are shared between both
versions and only use the "51DEGREES_" prefix.

The related CFLAGS and LDFLAGS can now be overridden using
51DEGREES_CFLAGS and 51DEGREES_LDFLAGS, both of which are automatically
collected into the respective OPTIONS_*. The USE_51DEGREES_V4 option is
now removed, and the doc was updated.