Mk/Uses/cabal.mk: Support for USE_CABAL in separate Makefile.cabal
Like we already do have in Mk/Uses/cargo.mk for Makefile.crates.
- Adopt www/hs-postgrest to this change in framework.
Reviewed by: vvd@
Approved by: portmgr@ (arrowd@)
Approved by: db@, yuri@ (Mentors, implicit)
Differential Revision: https://reviews.freebsd.org/D51936