Bug 139: portability fixes and documentation.

Document the dynamic lookup module capability in spec.xfpt.
Include a ChangeLog item.

Avoid the GNU-specific "export" make(1) directive.
Build the lookups Makefile using the existing framework.
Build with BSD Make once more.

The src/lookups/Makefile that is used at build time now has the dynamic
content come from scripts/lookups-Makefile.

Add CFLAGS_DYNAMIC support, which can be set in Local/Makefile.
Provide defaults for Linux & FreeBSD.

Ensure that build fails early if a dynamic module is requested but
CFLAGS_DYNAMIC is not defined.