Improve template builds

Additional projects in templates

To build a full system with minimal dependencies multiple projects are added.

Set common environment variables to allow custom prefixes

When installed into a custom prefix, autoconf and pkgconfig require certain environment variables to be set to this custom prefix to allow installation of subsequent projects that depend on the previous ones.

Close #56 (closed)

Run CI with custom prefix

We fix the CI by installing additional dependencies and compiling everything into a custom prefix, what is an important and most demanding use case to test.