Fixed a crash when building a subset of xcode6 targets

E.g. when running 'waf xcode6 --targets=some-target'

  File "/waf/waflib/Scripting.py", line 167, in waf_entry_point
    run_commands()
  File "/waf/waflib/Scripting.py", line 268, in run_commands
    ctx = run_command(cmd_name)
  File "/waf/waflib/Scripting.py", line 252, in run_command
    ctx.execute()
  File "/waf/waflib/extras/xcode6.py", line 679, in execute
    self.post_group()
  File "/waf/waflib/Build.py", line 767, in post_group
    if self.current_group < self._min_grp:
AttributeError: 'xcode' object has no attribute '_min_grp'

playground/xcode6/wscript

@@ -43,7 +43,7 @@ def configure(conf):
 	# This must be called at the end of configure()
 	conf.load('compiler_cxx xcode6')
 
-	conf.env.CXXFLAGS='-std=c++11'
+	conf.env.append_value('CXXFLAGS', ['-O2'])
 
 	conf.check(cxxflags='-std=c++11', uselib_store='STD11', mandatory=False)
 
@@ -106,7 +106,7 @@ def build(bld):
 		target='MyApp',
 		use='MyLib',
 		uselib='SDL2',
-		cxxflags='-O3',
+		cxxflags='-DSOME_DEFINE',
 		framework='Cocoa',
 		# Override default setting in a target
 		settings={"Debug": {"CONFIG_NAME": 'Debug'}}

waflib/extras/xcode6.py

@@ -681,6 +681,9 @@ class xcode(Build.BuildContext):
 
 		# post all task generators
 		# the process_xcode method above will be called for each target
+		if self.targets and self.targets != '*':
+			(self._min_grp, self._exact_tg) = self.get_targets()
+
 		self.current_group = 0
 		while self.current_group < len(self.groups):
 			self.post_group()