ninja/mac: allow path-based specification of SDKROOT

ninja/mac: allow path-based specification of SDKROOT

This matches the Xcode behavior of SDKROOT treatment. For SDKROOT values that
don’t begin with macosx, SDKROOT is treated as a path. When a relative path,
it is taken as relative to the .xcodeproj's containing directory, although
that isn't duplicated here because it's not likely to be of use. When SDKROOT
is an absolute path, it points to the root directory of the SDK itself, such
as /Developer/SDKs/MacOSX10.6.sdk or
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk .
Committed: https://code.google.com/p/gyp/source/detail?r=1440

[Mark Mentovai, 2012-07-26 21:47:42]

This enables https://chromiumcodereview.appspot.com/10831039/ to work for ninja.

I wanted to write a test for this but couldn’t identify a great way to do it
because there’s no longer really any standard for where SDKs might be located or
what SDKs might be available on a system.

[Nico, 2012-07-26 22:37:56]

lgtm

Matching xcode is good, independent of if we do the other CL.