Add hack to generate qt.conf after qtsvg

* configure.ac (qttranslations): Ensure it's built after qtsvg.
* src/Makefile.am (pkg_qtsvg_post_install): Generate qt.conf here.

For some reason I don't understand qmake does not add -lz to
the link line of qtsvg if the qt.conf exists when configuring
qtsvg. So as a workaround we create the file only after qtsvg.