[non-upstream qmake fix] Use pkgconfig in order to find libproxy configuration
=libproxy-0.5 moved proxy.h into a non-default include search path.
See also: https://github.com/libproxy/libproxy/issues/226#issuecomment-1557064225 https://bugs.gentoo.org/906879
Signed-off-by: Andreas Sturmlechner asturm@gentoo.org