Bug 1396730 - Log "checking watchman" before checking for watchman. r=chmanchester

When discussing bug 1395079 before it was filed, config.log didn't show
"checking watchman" while it should have. We fix this here. This also
makes it printed out in the configure output (obviously), but mach
buffers that, so when configure runs through mach, it doesn't actually
show up until the result is printed out, or, if the user interrupts
configure with CTRL+C (which is better than not showing up at all in the
latter case).

--HG--
extra : rebase_source : 11a2b5c497d7b9db3ac29cb34ff0ea2a90c179c9

moz.configure

@@ -371,7 +371,8 @@ tup = check_prog('TUP', tup_progs)
 option(env='WATCHMAN', nargs=1, help='Path to the watchman program')
 
 @depends('WATCHMAN')
-def watchman_info(prog):
+@checking('for watchman', callback=lambda w: w.path if w else 'not found')
+def watchman(prog):
     if not prog:
         prog = find_program('watchman')
 
@@ -388,17 +389,12 @@ def watchman_info(prog):
 
     return namespace(path=prog, version=Version(out.strip()))
 
-@depends_if(watchman_info)
-@checking('for watchman')
-def watchman(w):
-    return w.path
-
-@depends_if(watchman_info)
+@depends_if(watchman)
 @checking('for watchman version')
 def watchman_version(w):
     return w.version
 
-set_config('WATCHMAN', watchman)
+set_config('WATCHMAN', watchman.path)
 
 @depends_all(hg_version, hg_config, watchman)
 @checking('for watchman Mercurial integration')