isl: Update to 0.18

[ryandesign (Ryan Carsten Schmidt)]

Please update isl:

$ port livecheck isl
isl seems to have been updated (port version: 0.14.1, new version: 0.18)

gcc7 cannot be updated further until isl is updated to 0.15 or later.

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

In 4b59f760/macports-ports:

isl: update version 0.14.1->0.18

Library name has changed, so revbump dependencies.

Fixes #53617

[mojca (Mojca Miklavec)]

This broke gcc48. See #53662.

[mojca (Mojca Miklavec)]

In af91de7251fbd3f0c8b400bea541071d0a4ba59f/macports-ports:

*-w64-mingw32-gcc: revbump after isl update

Revbump cross-compilers after isl update:

• i686-w64-mingw32-gcc
• x86_64-w64-mingw32-gcc

See: #53617