Please backport r53400 to 2.3
r53400 fixes a bug in which
includedir, instead of
oldincludedir, is inappropriately prefixed with
SDKROOT. This bug was introduced in 2.3.0preview1, and fixed in 2.4.0preview1. If there will be a 2.3.5, please backport this to the 2.3 branch; it affects 2.3.4, and breaks building C extensions for some users.