astro/libnova: fix fetching, improve port
- Pull sources from Gentoo repo because upstream stopped tarball releases after v0.15
- Clarify LICENSE
- Patch to remove hardcoded AM_FLAGS
- Patch to fix linking with libelp
PR: 292368
Reported by: diizzy@
Approved by: db@, yuri@ (Mentors, implicit)
MFH: 2025Q4