Test each include file directly, instead of doing a large #include. This
verifies that each header is compilable on its own. One big advantage of
doing it via a special compiler in qmake is that we skip pre-compiled
headers, which has hidden build errors in the past.

This solution is implemented by making syncqt produce a second list of
headers.  This list is the same as the list of headers in the source
code to be installed, minus the headers that declare themselves to be
unclean, via the pragma:
  #pragma qt_sync_skip_header_check

This mechanism is applied only for public libraries (skipping
QtPlatformSupport, an internal_module).

This test is enabled only for -developer-builds of Qt because it
increases the compilation time.

On QtTest: the library only links to QtCore, but it has two headers that
provide inline-only functionality by including QtGui and QtWidgets
headers (namely, qtest_gui.h and qtest_widget.h). If those two modules
aren't getting compiled due to -no-gui or -no-widgets to configure, we
need to remove the respective headers from the list of headers to be
checked. If they are being built, then we need to make QtTest's build
wait for the headers to be generated and that happens when qmake is
first run inside the src/gui and src/widgets directories.

Change-Id: I57d64bd697a92367c8464c073a42e4d142a9a15f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>