Bug 1523198 - Refactor vc_compiler_path. r=froydnj

- Only expose it as well as --with-visual-studio-version when the host
  system is Windows.
- Don't run vswhere twice (once for host and once for target).

Depends on D17785

Differential Revision: https://phabricator.services.mozilla.com/D17786

--HG--
extra : moz-landing-system : lando

build/moz.configure/toolchain.configure

@@ -708,52 +708,55 @@ def get_vc_paths(topsrcdir):
         })
 
 
+@depends(host)
+def host_is_windows(host):
+    if host.kernel == 'WINNT':
+        return True
+
+
 js_option('--with-visual-studio-version', nargs=1,
-          choices=('2017',),
+          choices=('2017',), when=host_is_windows,
           help='Select a specific Visual Studio version to use')
 
 
-@depends('--with-visual-studio-version')
+@depends('--with-visual-studio-version', when=host_is_windows)
 def vs_major_version(value):
     if value:
         return {'2017': 15}[value[0]]
 
 
+@depends(vs_major_version, check_build_environment, '--with-visual-studio-version',
+         when=host_is_windows)
+@imports(_from='__builtin__', _import='sorted')
+@imports(_from='operator', _import='itemgetter')
+def vc_compiler_paths_for_version(vs_major_version, env, vs_release_name):
+    all_versions = sorted(get_vc_paths(env.topsrcdir), key=itemgetter(0))
+    if not all_versions:
+        return
+    if vs_major_version:
+        versions = [d for (v, d) in all_versions if v.major ==
+                    vs_major_version]
+        if not versions:
+            die('Visual Studio %s could not be found!' % vs_release_name)
+        return versions[0]
+    else:
+        # Choose the newest version.
+        return all_versions[-1][1]
+
 @template
 def vc_compiler_path_for(host_or_target):
-    @depends(host, host_or_target, vs_major_version, check_build_environment,
-             '--with-visual-studio-version')
-    @imports(_from='__builtin__', _import='sorted')
-    @imports(_from='operator', _import='itemgetter')
-    @imports('platform')
-    def vc_compiler_path(host, target, vs_major_version, env, vs_release_name):
-        if host.kernel != 'WINNT':
-            return
+    @depends(host_or_target, vc_compiler_paths_for_version,
+             when=host_is_windows)
+    def vc_compiler_path(target, paths):
         vc_target = {
             'x86': 'x86',
             'x86_64': 'x64',
             'arm': 'arm',
             'aarch64': 'arm64'
         }.get(target.cpu)
-        if vc_target is None:
-            return
-
-        all_versions = sorted(get_vc_paths(env.topsrcdir), key=itemgetter(0))
-        if not all_versions:
-            return
-        if vs_major_version:
-            versions = [d for (v, d) in all_versions if v.major ==
-                        vs_major_version]
-            if not versions:
-                die('Visual Studio %s could not be found!' % vs_release_name)
-            data = versions[0]
-        else:
-            # Choose the newest version.
-            data = all_versions[-1][1]
-        paths = data.get(vc_target)
         if not paths:
             return
-        return paths
+        return paths.get(vc_target)
     return vc_compiler_path