Guile ports: remove conflicts and add flavors

[andrew_tao173.riddles.org.uk]

Summary

Currently, the three Guile versions and their dependent ports conflict, preventing installation of more than one at a time. This is clearly unsatisfactory, so here is a fix for it.

What this does:

1. Creates USES=guile[:options] and updates ports to use it.

2. To help with conflicts over .pc files, USES=pkgconfig now allows you to specify PKGCONFIG_PATHS to allow management of PKG_CONFIG_PATHS in a cooperative fashion.

3. Some specific Guile library ports (guile-lib, guile-cairo, slib-guile) have been flavorized.

4. Currently none of the lang/guileN ports install a plain "guile", they install guile1, guile-1.8, guile2, guile-2.2, guile3, guile-3.0 respectively. This could be changed; should "guile" refer to the latest version, or the default version?

5. some PORTREVISIONs in this patch will be wrong, this will be re-checked after testing is complete

Test Plan

Tests done so far, all on amd64 unless stated:

• install all three Guile versions at once and test clients of each: lilypond for guile1, guile-gnome-platform for guile2, and guile-cairo (via g-golf which is not yet a port) for guile3.
• testport builds for all ports touched in the patch so far
• actual runtime tests on guile-gnome-platform, guile-cairo, g-wrap, guile-lib, lilypond (w/ and w/o GUILE2 option), lilypond-devel, irc/weechat, finance/gnucash, audio/denemo, games/aisleriot, games/gnurobots, www/elinks
• build guile3 on i386 to confirm it is not BROKEN as was previously specified

Tests planned:

• run tests on some remaining ports
• test on i386 and armv7 if resources allow (I don't have an aarch64 system to test on)
• test some non-clean builds to check that the correct Guile version is being picked up

[andrew_tao173.riddles.org.uk]

Update with more ports, and improved logic in guile.mk.

[andrew_tao173.riddles.org.uk]

Rebase.

Add remaining guile ports; this should now be complete.