math/z3: Update 4.15.3 => 4.15.4

Changelogs:

• https://github.com/Z3Prover/z3/blob/master/RELEASE_NOTES.md#version-4154
• https://github.com/Z3Prover/z3/compare/z3-4.15.3...z3-4.15.4

• Add WWW for main site
• Use dynamic SOVERSION for pkg-plist

Approved by: haskell@ (alven@, arrowd@)
Approved by: db@, yuri@ (Mentors, implicit)
Differential Revision: https://reviews.freebsd.org/D53474