Sets the `basedirlist` option in setup.cfg such that distutils does not search
for headers and libraries in `/usr`, and `/usr/local`.
Otherwise the build can fail on non-NixOS machines, see #15993.
In line with the Nixpkgs manual.
A mechanical change, done with this command:
find pkgs -name "*.nix" | \
while read f; do \
sed -e 's/description\s*=\s*"\([a-z]\)/description = "\u\1/' -i "$f"; \
done
I manually skipped some:
* Descriptions starting with an abbreviation, a user name or package name
* Frequently generated expressions (haskell-packages.nix)
