remove the --use-native-printer option for Mac OS X

This reverts commit fd60bd9cba2a8c6f9dd23a66efc293a2725a6c69,
which was added due to #1536. However, as per information
available in #1545 it appears that a fix is being made at the
Qt end, which means that it will no longer be needed.
sync public headers to remove the __WKHTMLTOX_UNDEF_QT_DLL__ hack

This should have been done in c22928d1517ca05ac0c6433272dc51d7d68aaed5.
clarify location of cloned repository for linux builds

In case the cloned repository is out of the home directory, starting
the schroot can fail with an error "Failed to change to directory".
use SUDO_USER if "logname" does not return any result

This can happen in certain situations e.g. when you enter an
OpenVZ container via "vzctl enter CTID" the login name is not
available.
add --force and --verbose to the XZ compression options

This will generate the output even if it already exists and show
additional information during the compression.
fix incorrect display of downloaded files in the build script

This was due to stdout not being flushed in 659a072fe6eb646523d8feba8147dbd6842cc417
ignore all warnings when building Qt for the Linux builds

We cannot do this for the local POSIX build as we don't know which
compiler-specific flags will disable it.
perform an in-place prefix build for Qt in the build script

This removes the need for installing the patched Qt.
add support for OS X in the build script

This was based on suggestions/feedback by @mn4367 and @npinchot
and fixes #1545
Merge remote-tracking branch 'upstream/master'

Conflicts:
	scripts/build.py