QtImageformats fails to compile because system libtiff was selected

The TIFF image handler uses type tmsize_t but it does not check whether the system libtiff does provide this type. Thus configure selects system libtiff but should have used the qt-shipped one.

This is especially bad, since it is not possible to override this via configure command line as it used to be. 

In config.test/libtiff.libtiff.cpp something like the following should be added:

tmsize_t ss=0;
(void)ss;