Log Message:
Misc pkgsrc tweaks for the latest jwm-2.3.6:

- LICENSE has been changed from GPLv2 to MIT since 2.3.6
- use proper ${PREFIX} in system.jwmrc for default icons
- add and enable svg option in options.mk for default icons
- explicitly pass --disable-foo to CONFIGURE_ARGS for disabled options

While here, avoid SUBST on post-patch for future mkpatches ops.

Bump PKGREVISION.