Build issue on MacOS: python 3.13, cython 3.2, clang 17

Build issues on MacOS with the following configuration, signaled by Chantal Pitte:

(lisa_new_env) chantalpitte@dhcp-022 phenomxpy % cython --version
Cython version 3.2.1
(lisa_new_env) chantalpitte@dhcp-022 phenomxpy % clang --version
Apple clang version 17.0.0 (clang-1700.0.13.5)
Target: arm64-apple-darwin24.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

I had to change every "inline" in spline.h, struct.h etc. for "static inline". For latest mac, latest cython, latest clang is needed that way otherwise they appear as empty symbols