devel/isl: Update URLs, fix build and connect test suite

Update URLs (old main site dead)
Fix port to follow CFLAGS set by framework instead of hardsetting -O3 only
Rearrange Makefile

Reference: https://groups.google.com/g/isl-development/c/JGaMo2VUu_8

Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D32494