Remove inline keyword, unrecognised by clang

Fix -pedantic build

Until now, freekdl has been building without the -pedantic flag which
caused gcc and clang not to respect some of the rules of ANSI C. I was
mistakenly using features not available yet. This change fixes that.

Add sourcehut build badge to README

Add source hut CI

Fix issues found by -Wall -Wextra

Remove use of min() macro in tests

FreeKDL initial public release