Correction to fontconfig usage on Unix-not-OSX.

The PhantomJS QPA hardcoded usage of QFontconfigDatabase on
Unix-not-OSX, causing build failures in "bundle all the libraries" mode.
Use QGenericUnixFontDatabase instead, which is QFontconfigDatabase if
fontconfig is enabled, and QBasicFontDatabase if it isn't.  This means
that the bundled-qtdeps build will use only the fonts bundled with Qt,
and won't be able to find them on a machine that doesn't have the source
tree, which is suboptimal, but at least this makes the bundled-qtdeps
build complete successfully again.

Part of issue #12467.