Cmake find_package does not work with Doxygen

Question

Objective

I am trying to have CMake find Doxygen on a bunch of different systems and configurations:

• Online documentation is generated on Ubuntu
• Local tests and development is done on Macos
• Doxygen can be installed system-wide, through brew or through conan depending on the use case

Problem

I struggle finding a uniform approach that does not involve a lot of OS-specific operations. I am vaguely tempted to glue together the 3 following approaches, but I am still surprised/concerned/unsure a more elegant solution does not exist.

Sub-solution 1

What I have presently works only for system-wide installations:

find_package(Doxygen
             REQUIRED dot
             OPTIONAL_COMPONENTS mscgen dia)

if(DOXYGEN_FOUND)

  # doxygen settings can be set here, prefixed with "DOXYGEN_"
  # ...

  # this target will only be built if specifically asked to.
  # run "make docs" to create the doxygen documentation
  doxygen_add_docs(
    docs
    ${PROJECT_SOURCE_DIR}
    COMMENT "Generate API-documents."
  )
else (DOXYGEN_FOUND)
  message([WARNING] " Doxygen need to be installed to generate the doxygen documentation")
endif (DOXYGEN_FOUND)

Sub-solution 2:

I am aware I can try to access Conan-installed executable through CONAN_BIN_DIRS_DOXYGEN/doxygen, as explained here

Sub-solution 3:

I am also aware I can try to locate Homebrew installations as described here.

Answer 1

Following @drodri comments, the following conan/conanfile.py achieves the desired result for my header-only library:

from conans import ConanFile, CMake
from conan.tools.cmake import CMakeDeps

class MyConan(ConanFile):
    name = "mylib"
    version = "0.1"
    settings = "os", "compiler", "arch", "build_type"
    exports_sources = "include/*", "CMakeLists.txt", "test/*", "cmake/*", "docs/*"
    no_copy_source = True
    generators = "cmake", "CMakeToolchain", "CMakeDeps"
    requires = "boost/[>=1.78.0]", "gdal/[>=3.4.3]"
    tool_requires = "cmake/3.24.2", "doxygen/1.9.4"

    def generate(self):
        cmake = CMakeDeps(self)
        # generate the config files for the tool require
        cmake.build_context_activated = ["doxygen/1.9.4"]
        cmake.generate()

    def build(self): # this is not building a library, just tests
        cmake = CMake(self)
        cmake.configure()
        cmake.build()
        cmake.test(output_on_failure=True)

    def package(self):
        self.copy("*.h")

    def package_id(self):
        self.info.clear()

It can be called with conan install conan/conanfile.py --build=missing --install-folder=build -pr:b=conan/profiles/clang_13 -pr:h=conan/profiles/clang_13