framework: reintroduce the feature enabling code

For each feature (see in Mk/Features) users can now set these variables
in their make.conf. Using LTO as an example.

To enable globally:

WITH_LTO= yes

To disable globally (for example ssp is enabled by default):

WITHOUT_LTO=	yes

To enable for a specific list of ports:

WITH_LTO_PORTS= lang/perl5.36

To disable for a specific list of ports:

WITHOUT_LTO_PORTS= lang/perl5.36

Differential Revision: https://reviews.freebsd.org/D44750