doxygen: add support for using libclang to parse files

Closes #20128.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
Luvit 0.6.1

Closes #19925.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
jenkins 1.519

Closes #20582.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
libslax 0.16.0

Closes #20560.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
postgis: style nits