gyp: don't print xcodebuild not found errors

As node-gyp rebuild doesn't seem to need xcodebuild, we don't need to be
printing the error every time GYP is run.

Refs: nodejs#1057
Refs: https://chromium-review.googlesource.com/c/492046/
Refs: nodejs#569

gyp/pylib/gyp/xcode_emulation.py

@@ -429,7 +429,7 @@ def _GetSdkVersionInfoItem(self, sdk, infoitem):
     # Since the CLT has no SDK paths anyway, returning None is the
     # most sensible route and should still do the right thing.
     try:
-      return GetStdout(['xcodebuild', '-version', '-sdk', sdk, infoitem])
+      return GetStdoutQuiet(['xcodebuild', '-version', '-sdk', sdk, infoitem])
     except:
       pass
 
@@ -1251,7 +1251,7 @@ def XcodeVersion():
   if XCODE_VERSION_CACHE:
     return XCODE_VERSION_CACHE
   try:
-    version_list = GetStdout(['xcodebuild', '-version']).splitlines()
+    version_list = GetStdoutQuiet(['xcodebuild', '-version']).splitlines()
     # In some circumstances xcodebuild exits 0 but doesn't return
     # the right results; for example, a user on 10.7 or 10.8 with
     # a bogus path set via xcode-select
@@ -1301,6 +1301,17 @@ def CLTVersion():
       continue
 
 
+def GetStdoutQuiet(cmdlist):
+  """Returns the content of standard output returned by invoking |cmdlist|.
+  Ignores the stderr.
+  Raises |GypError| if the command return with a non-zero return code."""
+  job = subprocess.Popen(cmdlist, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+  out = job.communicate()[0]
+  if job.returncode != 0:
+    raise GypError('Error %d running %s' % (job.returncode, cmdlist[0]))
+  return out.rstrip('\n')
+
+
 def GetStdout(cmdlist):
   """Returns the content of standard output returned by invoking |cmdlist|.
   Raises |GypError| if the command return with a non-zero return code."""