Implement imported target for FLANN

[SunBlack]

Base of documentation and coding style is shipped FindGTest.cmake script of CMake

[SunBlack]

Not sure if we really need -DFLANN_STATIC (just added it again, because I'm not sure)

[SergioRAgostinho]

LGTM. Thank you.

I'm curious to see if this will fix also #2802. I believe it will because we're using find_library regardless of the success of pkg-config and that will ensure that a full path to the library is passed on to the consuming targets. Ping @claudiofantacci to monitor this one.

[SergioRAgostinho]

Not sure if we really need -DFLANN_STATIC (just added it again, because I'm not sure)

I don't see an explicit use in our source files. But I have the impression this might be needed by FLANN and its dependencies. Actually that's the feeling that it comes out from this PR @jspricke submitted, specifically the addition of the LZ4_STATIC_FLAGS.

[claudiofantacci]

Ping @claudiofantacci to monitor this one.

Yes, I think it does. By a quicklook at the PR, it creates a FLANN target with all the required info, just as the CMake file I was suggesting to use to address the problem (StandardFindModule.cmake from YCM).

[SergioRAgostinho]

👍 Thanks.

I admit I was expecting a requirement to add FindFLANN.cmake to CMAKE_MODULE_PATH, but I suspect PCLConfig.cmake is doing that under the hood. Even better 🙌

Let's discuss squash strategy. I would propose to squash the first three commits and leave the last one separate.

[SunBlack]

Let's discuss squash strategy. I would propose to squash the first three commits and leave the last one separate.

I moved every changed in vfh_recognition/CMakeLists.txt to second commit, too ;).

[SergioRAgostinho]

I moved every changed in vfh_recognition/CMakeLists.txt to second commit, too ;).

Cool thanks 👍