Because Python's set type is unordered, storing include paths in it
can produce unnecessary re-builds by generating different compiler
command lines between successive builds. Avoid this by using the
sorted() function on the includes.
The documentation for Python ≥ 2.7 guarantees that sorted() is stable,
while for Python 2.5–2.6 it uses the same algorithm as list.sort(),
which is stable [1].
[1]: https://stackoverflow.com/a/1915418
