Tests which are expected to not build need to get a way to find the
Qt 5 config packages. Because they use try_compile, there is no way
to pass the contents to it.
Work around that by generating a file containing the prefix which
the tests will include.
Change-Id: If43080c241539e4af5fe1c183e7da72066278b73
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>