Update .gitignore for Qt 5.3.

 * Ignore src/phantomjs_plugin_import.cpp, which is now created during
   the build.
 * Add leading slashes to a bunch of files that should not be ignored if
   they crop up in subdirectories.

Part of issue #12467.
Better Qt version check.

 * Allow any patchlevel of Qt 5.3.x.
 * Do it in the master .pro file instead of main.cpp; this makes the build
   fail immediately rather than after compiling a bunch of stuff.

Part of issue #12467.
Add capability to build PhantomJS against system Qt/Webkit.

Invoking build.sh with --system-qtwebkit will skip the build of Qt and
QtWebkit and use the qmake in $PATH to build PhantomJS proper.

This doesn't actually *work* at the moment because the bundled
copy of Webkit has patches not yet merged upstream, but it's still
useful to have for testing purposes.

Part of issue #12467.
Rationalize handshake between build.sh and preconfig.sh.

 * build.sh now handles building qt and qtwebkit.
 * preconfig.sh is now only responsible for accumulating arguments
   to pass to qt's configure script.
 * preconfig.sh doesn't have command line arguments of its own;
   anything on its command line will be passed directly to qt's
   configure script.
 * first pass of adjustments to the qt configure parameters
   to try to get a more static build - unfortunately, system
   sqlite, libz, libpng, and libexpat are still getting pulled
   in somehow (mostly via fontconfig, I think).

Part of issue #12467.
Add option to build against system qtbase but bundled qtwebkit.

As with the system-qtwebkit option, this does not actually work at this
point, but it enables experimentation toward getting it to work.

Part of issue #12467.
Correct SQLITE3SRCDIR setting.

This is how we arrange for QtWebkit to use QtBase's bundled copy of
sqlite; without it, QtWebkit will attempt to use a system-provided
library instead, and if headers are unavailable, the build will fail.

Part of issue #12467.
Add --system-qtdeps build mode.

In this mode, system-provided libraries will be used for all of
Qt's dependencies, but Qt and QtWebkit themselves are still the
bundled copies.  This mode actually works.

Now that this mode exists, disable fontconfig in the "everything
bundled" mode, because it drags in the system freetype and several
other system libraries.  (There is no bundled fontconfig.)

Part of issue #12467.
80-column compliance for build.sh messages.

Cosmetic fix as part of issue #12467.