Meson build: improvements and fixes

This merge contains several small changes to fix bugs and tidy up the meson build files:

1. Add -Ofast to optimised builds, if supported
2. Improve function checking in main build file
3. Tidy up dependency checking
4. Include descriptions in generated config.h
5. Only try to build docs for cross-compile if we can run objects in build environment
6. Use new meson copy option for file configuration objects
7. Tidy up readme generation
8. Add new automake build tests to meson build
9. Improve lcms dependency handling
10. Add missing SSE2 compile flags to libbabl compile - fixes issue: #29 (closed)

The changes are all quite small and pass all the CI tests