perl: Disable tests

They're on the critical path for stdenv rebuilds.
bash: Update to 4.2-p47
util-linux: Update to 2.24.2
boost: Drop unnecessary runtime dependency on Python

The only file that actually referred to the Python interpreter was
share/boost-build/example/customization/inline_file.py. So get rid of
the whole example directory.
Remove the unmaintained (and AFAIK unused) stdenv for mingw
Another attempt to eradicate ensureDir

See c556a6ea46e71e1907d78b71fab36df30297b3ad.
Add a deprecation warning to ensureDir
gcc: Fix purity

Somewhere the no-sys-dirs.patch got disabled, so gcc was looking in
/usr/local/include and /usr/lib. Since I can't fix the patch easily,
I've borrowed the --sysroot trick from clang-wrapper. This causes
builtin paths to be prefixed with /var/empty
(e.g. /var/empty/usr/lib), which don't exist.
gcc: Slight builder cleanup
Darwin: Use xcrun to get the path to the SDK

Recent versions of Xcode don't install headers in /usr/include but
in a directory like

  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include

So use that instead, falling back to /usr/include in case of an older
version of Xcode.
Merge remote-tracking branch 'origin/master' into staging

Conflicts:
	pkgs/servers/serfdom/default.nix
gcc-4.8: Get rid of lib64

Having a separate lib64 is bad because it requires special-casing in
lots of places.

Previously done in e4a11b4d92424c42f61ee55cf505ac1217944f4a, which
apparently got lost going from 4.6 to 4.8.
readline: Don't use gcc-apple
Darwin: Search for libraries in $sdk/usr/lib

This required adding a new environment variable NIX_LDFLAGS_AFTER
that ensures that the -L.../usr/lib flag happens last.
Remove last reference to stdenvType
stdenv: Fix __ignoreNulls

Commit 262c21ed464a2593b54ec274a3b84083c0795f2d purported to enable
ignoreNulls, but it was bogus because it set the flag on the wrong
derivation (i.e. stdenv rather than the result of mkDerivation).
Fix SNAFU using $NIX_LDFLAGS_AFTER
Set MACOSX_DEPLOYMENT_TARGET

This variable sets the minimal Mac OS X version required for
running binaries produced by the Darwin toolchain. Since it
defaults to the version of the user's SDK, setting it explicitly
should make our builds more deterministic. It's now set to 10.6
because that's what hydra.nixos.org runs.
Partially merge gcc-wrapper and clang-wrapper

Having a separate clang-wrapper is really unfortunate because it
means that we'll forever forget to apply changes to both (e.g.
commit 289895fe2ceae2ade5aad635018eaeeaeaa669c6). This commit
gets rid of the redundant copies of ld-wrapper.sh and utils.sh.
gdk-pixbuf: check for empty $out in setup-hook.sh. Closes #2818