Add zlib v1.3.1 as submodule

[beutlich]

This introduces the zlib dependency as git submodule instead of copying the source files over.

Note, that 62848d3 introduced a change to "our" zlib which is not available in zlib upstream. If you see an issue here to revert that change we could also add the dependency from a custom fork, e.g. https://github.com/modelica/zlib.

Note, that the submodule is added via HTTPS, not SSH.

[HansOlsson]

make: *** No rule to make target '../../C-Sources/zlib/adler32.c', needed by '../../C-Sources/zlib/adler32.lo'. Stop.

[dietmarw]

@HansOlsson Did you remember to run git submodule init and git submodule update before trying to run make?

[HansOlsson]

@HansOlsson Did you remember to run git submodule init and git submodule update before trying to run make?

I will check that later.
However, the error message wasn't from my local machine, but copied from the failing CI.

I assume a similar procedure is needed there.

[dietmarw]

Tested with ./configure and make

[dietmarw]

@HansOlsson Did you remember to run git submodule init and git submodule update before trying to run make?

I will check that later. However, the error message wasn't from my local machine, but copied from the failing CI.

I assume a similar procedure is needed there.

Ah I see. OK I guess @beutlich will look into this later.

[beutlich]

make: *** No rule to make target '../../C-Sources/zlib/adler32.c', needed by '../../C-Sources/zlib/adler32.lo'. Stop.

Yes, alread noticed but it is not obvious how to fix the Jenkinsfile. @sjoelund Please advice.