When cross-compiling, the gcc and clang `--sysroot=` compiler option may be used to change the logical path of the system headers and libraries to a path located within the install path of the cross-compiler tool chain.

Android is a special case:
--------------------------
With Unified Headers [1], the most recent versions of the Android NDK add an additional constraint: the root paths of usr/include and usr/lib are distinct. For example when cross-compiling for API 24 and the x86_64 architecture the full paths are:
* headers:   ANDROID_NDK_ROOT/sysroot/usr/include
* libraries: ANDROID_NDK_ROOT/platforms/android-24/arch-x86_64/usr/lib64

To use sysroot in this example one needs to set sysroot to ANDROID_NDK_ROOT/sysroot in CFLAGS or CPPFLAGS and to set sysroot to ANDROID_NDK_ROOT/platforms/android-24/arch-x86_64 in LDFLAGS, while on a standard platform where the root path of usr/include and usr/lib is common, one would only need to set sysroot to this root path in CFLAGS.

[1] https://android.googlesource.com/platform/ndk.git/+/master/docs/UnifiedHeaders.md

> while on a standard platform where the root path of usr/include and usr/lib is common, one would only need to set sysroot to this root path in CFLAGS

s/CFLAGS/CC/

New changeset 77f5139954a878b856b0ac4c76486b27b6f4ec26 by xdegaye in branch 'master':
bpo-32059: setup.py now also searches the sysroot paths (GH-4452)
https://github.com/python/cpython/commit/77f5139954a878b856b0ac4c76486b27b6f4ec26

New changeset 04af8ace86d4396bc64fc401f36049ab745fd8c1 by xdegaye (Miss Islington (bot)) in branch '3.6':
bpo-32059: setup.py now also searches the sysroot paths (GH-4452) (#4562)
https://github.com/python/cpython/commit/04af8ace86d4396bc64fc401f36049ab745fd8c1