Make it possible to disable ALSA and PulseAudio in qtbase. (#12518)

Cherry-picked configure script changes from upstream Qt5.3.
More adjustments to system library usage on Unix (incl. Darwin) (#12518).

The affected libraries are fontconfig, freetype, harfbuzz, libpng, libjpeg,
sqlite, pcre, and zlib.

* On Darwin, by default, fontconfig is disabled and all of the other
  libraries are bundled.
* On non-Darwin, by default, fontconfig is enabled and system-provided
  freetype, libpng, libjpeg, and zlib are used.  Harfbuzz, pcre, and sqlite
  continue to be bundled.

* Individual libraries from the above list may now be enabled or disabled
  on the build.sh command line with e.g. --freetype=system or
  --freetype=bundled.

* The existing --system-qtdeps, --system-qt, --system-qtwebkit options
  have been renamed --qtdeps=system, --qt=system, --qtwebkit=system
  respectively.  New option --qtdeps=bundled forces all available
  bundled libraries to be used.

Also:

* Slight adjustments to organization of preconfig.sh.
* Can now disable PulseAudio and ALSA.
* Directly link OpenSSL instead of loading it at runtime.
* Clarify the pkg-config situation (it must be enabled if and only if
  sqlite comes from the system).
Explicitly suppress video and audio when building QtWebkit. (#12518)

This mostly matters when system sqlite is selected; that has the side
effect of enabling pkg-config which in turn causes QtWebkit to detect
the presence of system GStreamer and/or QtMultimedia and turn video on.
This way, the build is consistent across the board.
Merge branch 'config-tweaks-round-2'