qt5: disable for static-only builds

qt5: disable for static-only builds

Even though we have some specific code to support building Qt5 for
static-only configurations, it doesn't work. The first problem is that
our custom qmake.conf always passes -ldl, which makes a number of Qt5
config.tests fail at configure time. Once this problem is fixed by
removing -ldl from QMAKE_LIBS and adding it to QMAKE_LIBS_DYNLOAD
instead, the next problem is that the plugin infrastructure of Qt5
assumes that Linux has dynamic library support: the qlibrary_unix.cpp
file includes <dlfcn.h>, and the only condition for this file to not
be included is:

Until recently, building Qt5 statically was working because our C
library was not built static-only: it provided <dlfcn.h> and
libdl.so. But now that we have a really static only toolchain, Qt5 no
longer builds.

The easiest solution is to simply make Qt5 depend on dynamic library
support.

Fixes:

   http://autobuild.buildroot.net/results/538/538e0325adba9fabbe4ec8e550fbb6a7219f5e7a/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>