www/osrm-backend: unbundle sol2

• Remove the DEBUG option - it did nothing.
• Trie to unbundle the included sol2 library.
• Remove the dependency on Clang 18 for FreeBSD-Current.
• Bump PORTREVISION

PR: 283292
MFH: 2025Q1
(cherry picked from commit 2ae03d3a684e76d8ecc643a13104088bef26fbb7)