in the meanwhile I have found the solution.
qmake does not know where libraries and header files are located. Further I pass also the path of openssl as argument since I had to comment out the section about winlocal.prf in the qca-ossl.pro file.
So the command looks like:

qmake.exe "OPENSSL_PREFIX=C:/path/to/openssl" "INCLUDEPATH+=C:/path/to/qca/include/QtCrypto" "LIBS+=-LC:/path/to/qca/lib *-lqca2" (-lqcad2 for debug)

For those interested in building qca with VisualC++, I have a complete step by step description of the building process (qca-2.0.3 | qca-ossl-2.0.0-beta3 | openssl-1.0.1e | Visual Studio 2008 (VC++ 9). Just drop me a message.