Build fixes

Just two tweaks:

• updated SOVERSION for debian (by @smr99 in #7, which is the first PR to this project)
• more include files for building on macOS 26 with Apple Clang 17

First release on GitHub

Highlights of the changes since 2.1.5-10, which was released almost 17 years
ago, are:

• build fixes for modern compilers on Linux and macOS
• bug fixes, most importantly font rendering on 64 bit Linux
• CMake as a modern alternative to the existing Makefile build system