Add macOS framework major version

From https://github.com/KDE/qca/pull/3

Is major.minor, though could be just major, like Qt, Qwt, etc. For example:

FRAMEWORK_VERSION ${QCA_LIB_MAJOR_VERSION}

Creates framework paths like:

${QCA_LIBRARY_INSTALL_DIR}/qca-qt5.framework/Versions/2.1/
OR
${QCA_LIBRARY_INSTALL_DIR}/qca-qt5.framework/Versions/2/

Whether it should be major.minor or just major is relative to how the project handles versioning and API or SO breaks. Regardless, current path is probably too restrictive (kind of defeating one benefit of using frameworks):

${QCA_LIBRARY_INSTALL_DIR}/qca-qt5.framework/Versions/2.1.3/