Xcode 7 MACOSX_DEPLOYMENT_TARGET and SDK fixes

SDK 10.10 isn't something that exists for Xcode 7, so stop looking for
it and rely on MACOSX_DEPLOYMENT_TARGET instead.

See PR Homebrew/homebrew#50137 Yosemite build failure

Closes Homebrew/homebrew#50355.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

Library/Homebrew/extend/ENV/super.rb

@@ -67,7 +67,7 @@ module Superenv
     self["HOMEBREW_INCLUDE_PATHS"] = determine_include_paths
     self["HOMEBREW_LIBRARY_PATHS"] = determine_library_paths
 
-    if MacOS::Xcode.without_clt?
+    if MacOS::Xcode.without_clt? || (MacOS::Xcode.installed? && MacOS::Xcode.version.to_i >= 7)
       self["MACOSX_DEPLOYMENT_TARGET"] = MacOS.version.to_s
       self["SDKROOT"] = MacOS.sdk_path
     end

Library/Homebrew/os/mac.rb

@@ -90,19 +90,24 @@ module OS
     # If the requested SDK is not installed returns either:
     # a) The newest SDK (if any SDKs are available), or
     # b) nil
-    def sdk(v = version)
+    def sdk(v = nil)
       @locator ||= SDKLocator.new
       begin
-        @locator.sdk_for v
+        sdk = if v.nil?
+          Xcode.version.to_i >= 7 ? @locator.latest_sdk : @locator.sdk_for(version)
+        else
+          @locator.sdk_for v
+        end
       rescue SDKLocator::NoSDKError
         sdk = @locator.latest_sdk
-        # don't return an SDK that's older than the OS version
-        sdk unless sdk.nil? || sdk.version < version
+      ensure
+        # only return an SDK older than the OS version if it was specifically requested
+        sdk if v || (!sdk.nil? && sdk.version >= version)
       end
     end
 
     # Returns the path to an SDK or nil, following the rules set by #sdk.
-    def sdk_path(v = version)
+    def sdk_path(v = nil)
       s = sdk(v)
       s.path unless s.nil?
     end