Simplify out-of-source CMake build
    
    The %cmake macro was updated[1] to specify the out-of-source build folder with -B %{_vpath_builddir}, so update our out-of-source build to use %{_vpath_builddir}.
    
     * libpqxx.spec:
        - Update %build, %install, and %check sections to use updated %cmake macro with %{_vpath_builddir} out-of-source build directory.
        - Update %check section to use ctest args consistent with %ctest macro.
    
    [1] https://fedoraproject.org/wiki/Changes/CMake_to_do_out-of-source_builds