xcode_emulation: work in the absence of xcodebuild

OS X systems running only the Command Line Tools for Xcode package,
without the full Xcode, don't have a functioning xcodebuild, but this
isn't mandatory for building many gyp projects (e.g. node.js, v8).
This commit handles xcodebuild failures and avoids populating
Xcode-specific CFLAGS/LDFLAGS when xcodebuild can't be run.

This has been tested on both Xcode and CLT-only systems by successfully
building node.js. The behaviour can be simulated on systems with Xcode
by setting the xcode-select path to something nonsensical, e.g.
xcode-select -switch /usr/bin

BUG=https://code.google.com/p/gyp/issues/detail?id=292
Review URL: https://codereview.chromium.org/102733012/

Patch from [email protected]!

This was a manual cherry-pick from:

  https://code.google.com/p/gyp/source/detail?r=1819

Fixes nodejs#341.

gyp/pylib/gyp/xcode_emulation.py

@@ -273,7 +273,14 @@ def _GetStdout(self, cmdlist):
     return out.rstrip('\n')
 
   def _GetSdkVersionInfoItem(self, sdk, infoitem):
-    return self._GetStdout(['xcodebuild', '-version', '-sdk', sdk, infoitem])
+    # xcodebuild requires Xcode and can't run on Command Line Tools-only
+    # systems from 10.7 onward.
+    # Since the CLT has no SDK paths anyway, returning None is the
+    # most sensible route and should still do the right thing.
+    try:
+      return self._GetStdout(['xcodebuild', '-version', '-sdk', sdk, infoitem])
+    except:
+      pass
 
   def _SdkRoot(self, configname):
     if configname is None:
@@ -397,10 +404,11 @@ def GetCflags(self, configname, arch=None):
 
     cflags += self._Settings().get('WARNING_CFLAGS', [])
 
-    config = self.spec['configurations'][self.configname]
-    framework_dirs = config.get('mac_framework_dirs', [])
-    for directory in framework_dirs:
-      cflags.append('-F' + directory.replace('$(SDKROOT)', sdk_root))
+    if 'SDKROOT' in self._Settings():
+      config = self.spec['configurations'][self.configname]
+      framework_dirs = config.get('mac_framework_dirs', [])
+      for directory in framework_dirs:
+        cflags.append('-F' + directory.replace('$(SDKROOT)', sdk_root))
 
     self.configname = None
     return cflags
@@ -647,10 +655,11 @@ def GetLdflags(self, configname, product_dir, gyp_to_build_path, arch=None):
     for rpath in self._Settings().get('LD_RUNPATH_SEARCH_PATHS', []):
       ldflags.append('-Wl,-rpath,' + rpath)
 
-    config = self.spec['configurations'][self.configname]
-    framework_dirs = config.get('mac_framework_dirs', [])
-    for directory in framework_dirs:
-      ldflags.append('-F' + directory.replace('$(SDKROOT)', self._SdkPath()))
+    if 'SDKROOT' in self._Settings():
+      config = self.spec['configurations'][self.configname]
+      framework_dirs = config.get('mac_framework_dirs', [])
+      for directory in framework_dirs:
+        ldflags.append('-F' + directory.replace('$(SDKROOT)', self._SdkPath()))
 
     self.configname = None
     return ldflags
@@ -826,7 +835,10 @@ def _AdjustLibrary(self, library, config_name=None):
         l = '-l' + m.group(1)
       else:
         l = library
-    return l.replace('$(SDKROOT)', self._SdkPath(config_name))
+    if self._SdkPath():
+      return l.replace('$(SDKROOT)', self._SdkPath(config_name))
+    else:
+      return l
 
   def AdjustLibraries(self, libraries, config_name=None):
     """Transforms entries like 'Cocoa.framework' in libraries into entries like