Previously several additional rules were added by qmake to build the required
libraries. This has a couple of problems on Windows, requiring the use of a
msys shell.
Additional variables are passed as environment variables. Windows doesn't
understand this. Passing the variables via make command line argument works but
make won't be able to change those variables anymore, breaking things. Use
intermediate variables passed via make command line that are then exported by
the called Makefile. This doesn't work in the generated Makefile since the
export lines must not be part of a rule.
Also cleans up the project file a bit.
Change-Id: Iaffb059ce8bafe17a35bd1e0a74ae7acd966c138
