These regex changes fix a number of issues for distutils on Windows:
- #6884: impossible to include a file starting with 'build'
- #9691 and #14004: sdist includes too many files
- #13193: test_filelist failures

This commit replaces the incorrect changes done in 557a973709de,
c566a3447ba1 and 3925081a7ca0 to fix #13193; we were too eager to fix
the test failures and I did not study the code enough before
greenlighting patches.  This time we have unit tests from the problems
reported by users to be sure we have the right fix.

Thanks to Nadeem Vawda for his help.

using --with-system-expat working when the system expat does not have
salted hash support.

in the hash table internal to the pyexpat module's copy of the expat
library to avoid a denial of service due to hash collisions.
Patch by David Malcolm with some modifications by the expat project.

    http://hg.python.org/cpython/rev/48705250232c
    changeset:   75187:48705250232c
    branch:      2.7
    parent:      75184:9a1d902714ae
    user:        Antoine Pitrou <solipsis@pitrou.net>
    date:        Wed Feb 22 22:16:25 2012 +0100

    changeset:   75153:9b7c6dd19e25
    branch:      2.7
    parent:      75151:b1a02c17b327
    user:        Antoine Pitrou <solipsis@pitrou.net>
    date:        Tue Feb 21 22:02:04 2012 +0100
    files:       Lib/test/test_os.py

    changeset:   75165:780008020c40
    user:        Antoine Pitrou <solipsis@pitrou.net>
    date:        Wed Feb 22 03:33:56 2012 +0100
    summary:     Fix (presumably) test_hash under big-endian systems (PPC).

Patch by Francisco Martín Brugué