sane-backends 1.0.21 and variants to limit which backends are built

[aguynamedryan+macports@…]

I have made the following updates to the sane-backends Portfile

• updated to build sane-backends 1.0.21
  • set revision to 0
  • updated checksums
• listed myself as the maintainer
• set use_parallel_build to yes

I've made the following enhancement:

I noticed that the command:

scanimage -L
}}} runs very slowly and the debug messages from SANE_DEBUG_DLL seem to indicate that scanimage takes a long time to run through each of the 75 backends when it searches for scanners.

It is possible to build sane-backends with a subset of the backends by setting the environment variable BACKENDS to the list of backends you'd like to compile before you run ./configure.

Accordingly, I made variants for the 75 backends in the sane-backends Portfile so a user can now run install like so:
{{{
port install sane-backends +net +hp +fujitsu
}}}
to build only the net, hp, and fujitsu backends.  This made scanadf run faster for me since it had fewer backends to iterate through.  As a bonus, compilation also takes far less time if just a few backends are specified.

I'm new to TCL and I'm not sure the way I specified the variants is particularly efficient, but it works.  If there is a better way to structure the variants in the Portfile I'd love to learn.

[aguynamedryan+macports@…]

update to sane-backends 1.0.21 and add 75 variants to specify subset of backends to build