Support system provided tidy-html and md4c

Hi,

When building Jami, even with the md4c and tidy-html packages available in the environment (pre-built), the build system ignores them and wants to use its submodule:

CMake Error at CMakeLists.txt:549 (add_subdirectory):
  The source directory

    /tmp/guix-build-jami-20231222.2.drv-0/source/3rdparty/md4c

  does not contain a CMakeLists.txt file.


CMake Error at CMakeLists.txt:557 (add_subdirectory):
  The source directory

    /tmp/guix-build-jami-20231222.2.drv-0/source/3rdparty/tidy-html5

  does not contain a CMakeLists.txt file.

It'd be nicer to add support to detect these from the system; they are packaged at least in GNU Guix (see: https://packages.guix.gnu.org/packages/tidy-html/5.8.0/ and https://packages.guix.gnu.org/packages/md4c/0.4.8/).