Make `PerMachine` and `MachineChoice` have just `build` and `host`

Meson itself *almost* only cares about the build and host platforms. The
exception is it takes a `target_machine` in the cross file and exposes
it to the user; but it doesn't do anything else with it. It's therefore
overkill to put target in `PerMachine` and `MachineChoice`. Instead, we
make a `PerThreeMachine` only for the machine infos.

Additionally fix a few other things that were bugging me in the process:

 - Get rid of `MachineInfos` class. Since `envconfig.py` was created, it
   has no methods that couldn't just got on `PerMachine`

 - Make `default_missing` and `miss_defaulting` work functionally. That
   means we can just locally bind rather than bind as class vars the
   "unfrozen" configuration. This helps prevent bugs where one forgets
   to freeze a configuration.

1 files changed, 1 insertions, 1 deletions

diff --git a/run_unittests.py b/run_unittests.py
index f4d969c..7242a5f 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -848,7 +848,7 @@ class InternalTests(unittest.TestCase):
                 PkgConfigDependency.check_pkgconfig = old_check
                 # Reset dependency class to ensure that in-process configure doesn't mess up
                 PkgConfigDependency.pkgbin_cache = {}
-                PkgConfigDependency.class_pkgbin = PerMachine(None, None, None)
+                PkgConfigDependency.class_pkgbin = PerMachine(None, None)
 
     def test_version_compare(self):
         comparefunc = mesonbuild.mesonlib.version_compare_many