mcompile: provide user visibility into what actually happens

This command is magical and I hate it. Mostly because it seems people
have no clue what it does, and what it doesn't do.

Provide informational messages to the user indicating how it works, e.g.
for debugging. Point out if we ran vsenv before shelling out to the
backend.

mesonbuild/mcompile.py

@@ -22,11 +22,12 @@ import shutil
 import typing as T
 from collections import defaultdict
 from pathlib import Path
+from time import sleep
 
 from . import mlog
 from . import mesonlib
 from . import coredata
-from .mesonlib import MesonException, RealPathAction, setup_vsenv
+from .mesonlib import MesonException, RealPathAction, join_args, setup_vsenv
 from mesonbuild.environment import detect_ninja
 from mesonbuild.coredata import UserArrayOption
 from mesonbuild import build
@@ -336,13 +337,16 @@ def run(options: 'argparse.Namespace') -> int:
 
     cdata = coredata.load(options.wd)
     b = build.load(options.wd)
-    setup_vsenv(b.need_vsenv)
+    vsenv_active = setup_vsenv(b.need_vsenv)
+    if vsenv_active:
+        mlog.log(mlog.green('INFO:'), 'automatically activated MSVC compiler environment')
 
     cmd = []    # type: T.List[str]
     env = None  # type: T.Optional[T.Dict[str, str]]
 
     backend = cdata.get_option(mesonlib.OptionKey('backend'))
     assert isinstance(backend, str)
+    mlog.log(mlog.green('INFO:'), 'autodetecting backend as', backend)
     if backend == 'ninja':
         cmd, env = get_parsed_args_ninja(options, bdir)
     elif backend.startswith('vs'):
@@ -353,6 +357,8 @@ def run(options: 'argparse.Namespace') -> int:
         raise MesonException(
             f'Backend `{backend}` is not yet supported by `compile`. Use generated project files directly instead.')
 
+    mlog.log(mlog.green('INFO:'), 'calculating backend command to run:', join_args(cmd))
+    sleep(2)
     p, *_ = mesonlib.Popen_safe(cmd, stdout=sys.stdout.buffer, stderr=sys.stderr.buffer, env=env)
 
     return p.returncode