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
Merge branch 'osx-support' of https://github.com/wkhtmltopdf/wkhtmltopdf into osx-support

Conflicts:
	INSTALL.md
	scripts/build.py