meson, autotools, CI: simplify plug-in binding build options.

For Python, Lua and Javascript, make the option boolean (with 'yes'
being the default). No need of a warning when not installing the
plug-ins as this would have been disabled explicitly anyway. When
installing the plug-ins, only make interpreter checks as precautionnary
verifications which don't actually change anything (except outputting
some warnings if interpreters are not found). Basically for these 3
bindings, the interpreters are only runtime dependencies anyway. So it
doesn't matter if they are not available at build time. In particular,
we get rid of the 'force' option.

Vala rules do not change as the vala compiler is indeed needed at build
time and current checks work correctly. I just add a "Vala plug-ins"
line in the summary message of the meson configuration, as it was
missing.
16 jobs for master in 37 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
passed #936977
build-image

00:02:25

passed #936979
build-image-win32

00:02:12

passed #936978
build-image-win64

00:02:05

 
  Dependencies
passed #936980
deps-debian

00:01:45

passed #936982
deps-win32

00:10:18

passed #936981
deps-win64

00:13:11

 
  Gimp
passed #936983
gimp-autotools-debian

00:09:42

passed #936986
gimp-clang-debian

00:08:36

passed #936984
gimp-distcheck-debian

00:23:58

passed #936985
gimp-meson-debian

00:12:08

passed #936988
gimp-win32

00:09:38

passed #936987
gimp-win64

00:12:32

 
  Analysis
passed #936989
cppcheck

00:02:15

 
  Distribution
passed #936990
sources

00:03:49

passed #936992
win32-nightly

00:12:46

passed #936991
win64-nightly

00:09:10