Use CMake for building

[suve]

The current setup with a hand-written configure script that generates a Makefile is quite rudimentary, unfriendly for newcomers and very hard to integrate into other build systems (most notably Gradle, which is used for Android builds).

Migrating the game to CMake seems like a good idea.

• It is a popular and well-understood build system (even if it has its own flaws).
• CMake integrates nicely with Gradle. This would make it possible to simplify Android builds so that everything can be done via Gradle / Android Studio, without the need for the current hodgepodge of shell scripts.

Foreseeable problems:

• CMake, out-of-the-box, does not have Pascal support. Some extra CMake scripting would be needed for adding Pascal support.
• Support for different target OS / processors is needed. Currently, this is handled by simply passing some flags to the configure script, e.g. --flags="-Tandroid -Paarch64" when building for Android on 64-bit ARM.
• Apart from code compilation, the build process also includes asset optimization. This would also need to be preserved in the CMake scripting.