math/libbraiding: Improve port

• Use upstream release archive (removes USES= autoreconf and patch file)
• Change USES= compiler:c++11-lang to compiler:c++17-lang
• Use USE_CXXSTD helper instead of CXXFLAGS
• Remove USES= pathfix (doesn't work)
• Use --with-pkgconfigdir configure switch

PR: 284092
Approved by: portmgr (maintainer timeout, 2+ weeks)