A small experiment shows that it is not necessary to cache the DOXYGEN_PACKAGE DOXYGEN_DOT_PACKAGE and the RPM_MAKE_APIDOC variables.
Eric, I have checked in your patch plus the changes to remove the above mentioned variables from the cache.
This brings make/FindDoxygen.cmake almost back to what it was in revision 1.4 (minus a logic bug for setting the DOXYGEN_DOT_PACKAGE variable and minus the DOXYGEN_DOT_PATH variable).

Hi Robert,

Thanks for the fix.
Some remark about CMake usage and
possible second pass fix.

You shouldn't
INCLUDE(FindDoxygen)
but use
FIND_PACKAGE(Doxygen)

there are several mechanisms involved
in FIND_PACKAGE which are not handled
when using INCLUDE.
(see cmake --help-command FIND_PACKAGE
  or cmake --help-command INCLUDE)

Concerning the the MAKE_APIDOC add
I think it shouldn't be in FindDoxygen.cmake
but in our main CMakeLists.txt
In fact FindDoxygen.cmake included in tsp source
is a (modified) derivative taken from CMake distros.
I did submit an update to CMake team and will be pleased
to remove the TSP one as soon as CMake embbed
the new one, this would make 1 file less to maintain :=)

Moreover in our CMakeLists.txt there is
the
OPTION(BUILD_API_DOC)
which indicates whether if current build should
build doxygen doc.

Thus, I propose to rename MAKE_APIDOC
to RPM_MAKE_APIDOC.
This would be more obvious that this var will
be used by generated RPM spec file.

This way you may have build tree for
which BUILD_API_DOC is false but
when you
make tsp_rpm
the APIDOC rpm will be generated if doxygen+dot
were found.

I have attached my proposed patch.
I may commit I you are Ok with my proposal.

(file #14326)

The trouble was that
DOXYGEN_PACKAGE and DOXYGEN_DOT_PACKAGE
were not define as CACHE CMake variables in
make/FindDoxygen.cmake.

thus when the RPMTools_ADD_RPM_TARGETS is called
[ macro defined in make/UseRPMTools.cmake
  and used in main CMakeLists.txt          ]

the variables weren't defined anymore because
FindDoxygen.cmake was called in a subdir (src/doc)
and not in the main CMakeLists.txt.

In CMakeLists.txt when you recurse in subdir
you inherit the var definition from the parent
but parent does not inherit var definition from
subdir unless you set those vars as CACHE var.

I did replace
SET(DOXYGEN_PACKAGE doxygen)
with
SET(DOXYGEN_PACKAGE doxygen CACHE STRING "The rpm package name containing doxygen")

in make/FindDoxygen.cmake

[same for DOXYGEN_DOT_PACKAGE]

I've checked in the file.
You may update, it should work now.
You'll have to explicitely rerun cmake in order to
make thoses vars appears in CMakeCache.txt.

I agree with your solution of
%if statements around the %package apidoc

Ah it is already a build dependency, but a conditional one.
So the fix is to add a couple of %if statements around the %package apidoc bit.
Let me see what I can do here.