[cmake-developers] Built-in tag support for FindDoxygen

[Robert Dailey]

Doxygen supports linking external documentation together:
https://www.stack.nl/~dimitri/doxygen/manual/external.html

Using doxygen_add_docs(), it doesn't provide built-in support for tag
files. I'm thinking this would be beneficial since the way the
function is designed encourages modular documentation. At least, I
have my projects structured like this (targets):

A
A_doxygen
B
B_doxygen
C
C_doxygen

I have 1 doxygen target per real library target. And each library has
dependencies on others. When library C uses something from A, I want
C_doxygen to link to the tagfile generated by A_doxygen.

At the moment I'm accomplishing this by adding a target property to
each real target to keep track of target dependencies. Example:

add_library(C ...)
target_link_libraries(C A)
set_property(TARGET C PROPERTY TARGET_DEPENDENCIES A)

When I'm building A_doxygen target (using doxygen_add_docs()), I
specify DOXYGEN_GENERATE_TAGFILE. Then in C_doxygen, I specify
DOXYGEN_TAGFILES to point to the one output by A_doxygen.

I don't like keeping target properties to query the dependency tree,
but this is the best I could come up with. Is there value in
incorporating this into FindDoxygen.cmake? If so, I'd like to
contribute it, if it's useful.

Would love feedback and advice on this!