This way, it's possible to tell which applications and libraries depend
on the Qt private API and of which Qt library. Linux distributions can
use this information to decide which applications need to be recompiled
every time Qt itself is rebuilt.

This is done by scanning all class and struct definitions in the private
headers (we've already got the list from syncqt). I opted to add a new
script instead of modifying syncqt because then this can run in parallel
with the rest of the compilation, as opposed to during qmake
time. Another advantage is that it catches modifications to the headers
in between qmake executions.

Since this is already Unix specific, it should be no problem to use Perl.

This solution is limited to use of non-inline symbols of classes
declared in private headers. It will not catch free variables (such as
qsimd_p.h's qt_cpu_features), use of inlined functions or just plain use
of a class/struct for accessing its data members. However, this is
already better than nothing and should help Linux distributions quite a
lot. And there's no way to catch the latter issue anyway.

Change-Id: I049a653beeb5454c9539ffff13e3fff36400ebbd
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>