build: Add CMake option to use external expat

[uilianries]

Hello all!

This PR adds the CMake option FMILIB_SYSTEM_EXPAT, to be able to consume expat from system, provided by a package manager.

The official CMake module for Expat is FindEXPAT and provides both EXPAT_INCLUDE_DIRS and EXPAT_LIBRARIES.

By default, this option is disabled, so it should not break users.

The entire build log using the system expat is here:

fmilib-system-expat-build.log

It's possible to see:

-- Found EXPAT: /usr/lib/x86_64-linux-gnu/libexpat.so (found version "2.1.0")

Related to #100

[modelonrobinandersson]

Hi @uilianries and thank you for your PR. There is just something I have to check on my end, I will get back to you on Monday again.

[uilianries]

@modelonrobinandersson Thank you! Please, take your time to validate this change, I don't want to break users 😅

[modelonrobinandersson]

Hi @uilianries, I just need a few more days as I am trying to sort something out on my end, I will be back as soon as possible.

[modelonrobinandersson]

Hi again @uilianries, in order to accept contributions to FMIL there is the following requirement: https://github.com/modelon-community/contributing
I will only be able to merge your request after this process has been finished, let me know if you have any questions.

[uilianries]

@modelonrobinandersson Hello, is it possible to send electronic signed instead? I don't have a printer.

[modelonrobinandersson]

@modelonrobinandersson Hello, is it possible to send electronic signed instead? I don't have a printer.

Hi! Yes that works as well.

[uilianries]

@modelonrobinandersson Hello again, I just sent the email for the CLA, but there is not technical issue on Modelon side. I opened a separated issue reporting the case: modelon-community/contributing#2

[filip-stenstrom]

To be clear, I will review this, but need to finish some other work first. Will look at this when I can.

[filip-stenstrom]

Good addition. FMIL should have better dependency management and this is one step in that direction.

Things were made a bit complicated since we already have e.g. FMILIB_FIND_PACKAGE_ZLIB which behaves slightly different (bundles zlib with FMIL). But we think not doing that (like done here), and instead listing dependencies is the way forward.

I've added suggestions for most changes.

Thanks for contributing, and sorry for delay in reviewing.

[uilianries]

@filip-stenstrom Thank you for your review. All committed all your suggestions! Please, review again when possible. Don't worry about the delay, as maintainer of open source project too, I totally understand 😉

[filip-stenstrom]

Thanks, all good now. Merging.

[jschueller]

hi @uilianries, do you plan to do zlib/minizip too ?

[uilianries]

@jschueller That's possible.

[jschueller]

that would be great!