Static-plugin support fails with Qt Assistant

The first failure is non-fatal: qmake is run on assistant.pro, and the test if the qjpeg plugin exists fails as the plugin hasn't been built yet. This leaves the build of Assistant without JPEG support.

The second bug is fatal: if a binary installation (shared, and including qjpeg4.lib/qjpeg4.dll) is reconfigured as static, and if the static plugins are built before qmake processes assistant.pro, then qmake will try to link against qjpeg4.lib instead of the static plugin qjpeg.lib, and assistant.exe will not link due to unresolved external symbols.

The workaround is to remove all shared plugins before building a static build of Qt Assistant, or even better to install the Qt source package into a separate directory.