Add support for finding several dependencies without pkg-config

[traversaro]

In particular, add a dedicated ign_manual_search CMake macro, and then use it in FindUUID (fix #64) and in the find scripts for FFMPEG libraries.

[scpeters]

we are also calling find_path and find_library in several other find modules:

$ grep -l find_library $(grep -rlI find_path cmake/Find*) | sort
cmake/FindDL.cmake
cmake/FindFreeImage.cmake
cmake/FindGTS.cmake
cmake/FindIgnOGRE2.cmake
cmake/FindJSONCPP.cmake
cmake/FindODE.cmake
cmake/FindOptiX.cmake
cmake/FindSQLite3.cmake
cmake/FindTINYXML2.cmake
cmake/FindYAML.cmake
cmake/FindZIP.cmake

I suspect that we could use ign_manual_search in these find modules, though it's not needed for this PR.

[traversaro]

we are also calling find_path and find_library in several other find modules:

$ grep -l find_library $(grep -rlI find_path cmake/Find*) | sort
cmake/FindDL.cmake
cmake/FindFreeImage.cmake
cmake/FindGTS.cmake
cmake/FindIgnOGRE2.cmake
cmake/FindJSONCPP.cmake
cmake/FindODE.cmake
cmake/FindOptiX.cmake
cmake/FindSQLite3.cmake
cmake/FindTINYXML2.cmake
cmake/FindYAML.cmake
cmake/FindZIP.cmake

I suspect that we could use ign_manual_search in these find modules, though it's not needed for this PR.

Yes, a few of those would probably need to expose some other parameters in the ign_manual_search API, but for the moment I preferred to have the minimal changes necessary for microsoft/vcpkg#11273 (comment) .

[mjcarroll]

@traversaro Can you bring this up to date so we can land it?

[traversaro]

@traversaro Can you bring this up to date so we can land it?

Done!