Add Gitlab CI file for runnig CI tests with Linux

Tuukka Pasanenrequested to merge
illuusio/bsddialog:add-ci-runners into main

As I very much know that bsddialog(1) is targeted to BSD family OS and it's not very popular in Linux world. Still this PR adds CI targets for building it on gcc-image which probably is Debian, Alpine latest as it has Musl C-library (and there is one commit to fix building on Alpine). and Fedora which is about latest Glibc and GCC. In Alpine linux stage there is also build with Clang.

Using CI makes sure that every commit and PR will get tested that they compile and does not break compiling. In future CI should/can be added to have some tests also.

If CI feels too much then only 480b16c9d44ac7aa428a316c2114927c2fcaddb3 could be cherry picked to swipe out last of u_int or it can be separated to another MR.

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