Use generate_export_headers to fix static windows build
Building and linking a static qca library is tricky at the moment.
It relies on an undocumented define QCA_STATIC
that needs to be in place when building qca and when building the application that uses qca.
This patch proposes to generate a header file (qca_export.h) at build time that comes with the required information to export symbols when required using standard cmake functionality.