[qtbase] use the vcpkg-provided md4c library

[jobor]

The qtbase port uses the md4c library that's bundled with Qt's sources.

ports/qtbase/portfile.cmake has

    elseif(_input STREQUAL "libmd4c")
        string(APPEND INPUT_OPTIONS qt) # libmd4c is not yet in VCPKG (but required by qtdeclarative)

By now, we have a md4c vcpkg port: https://vcpkg.link/ports/md4c

Proposed solution

I suggest to use the vcpkg port instead of Qt's bundled sources.