cmake find modules wrt version - build

Question

I have a question regarding CMake: recently a lot of modules have been added to the standard installation, such as GLEW or Mercurial.
However a lot of installation base might have an older version without all the new modules available, so you are forced to ship your own version of (eg) FindGLEW.cmake
Is it possible to check whether a given FindXXX module is available and in that case use it, otherwise supply a proper alternative? Or even check cmake version at runtime (but that's not always reliable and a pain to maintain)...?
Thanks for any help.

Answer 1

The default search behaviour of CMake's include command w.r.t modules is:
the file with name <modulename>.cmake is searched first in CMAKE_MODULE_PATH, then in the CMake module directory.
(there's a bit more to it - see the docs or run cmake --help-command include)
What you're asking for appears to be the opposite to this; first check for an official module in the CMake module directory, and if none exists, fall back to use your own.
Assuming your modules are in ${CMAKE_SOURCE_DIR}/my_cmake_modules, you could reverse CMake's default behaviour by doing:
set(CMAKE_MODULE_PATH ${CMAKE_ROOT}/Modules ${CMAKE_SOURCE_DIR}/my_cmake_modules)
If you name your own modules exactly as per the official ones, then this should achieve your goal. If you wish to revert to the normal CMake behaviour later in your script, you can do:
set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/my_cmake_modules)

Answer 2

as a possible alternative to the solution above, I found this cmake policy
CMP0017
Prefer files from the CMake module directory when including from
there.
Starting with CMake 2.8.4, if a cmake-module shipped with CMake (i.e.
located in the CMake module directory) calls include() or
find_package(), the files located in the the CMake module directory
are preferred over the files in CMAKE_MODULE_PATH. This makes sure
that the modules belonging to CMake always get those files included
which they expect, and against which they were developed and tested.
In call other cases, the files found in CMAKE_MODULE_PATH still take
precedence over the ones in the CMake module directory. The OLD
behaviour is to always prefer files from CMAKE_MODULE_PATH over files
from the CMake modules directory.
This policy was introduced in CMake version 2.8.4. CMake version
2.8.9 warns when the policy is not set and uses OLD behavior. Use the
cmake_policy command to set it to OLD or NEW explicitly.
Hope this helps.