ecl_16_1_2: fix linking of compiled programs
ecl_16_1_2: fix linking of compiled programs

* Use --with-gmp-incdir and --with-gmp-libdir (like for the main ecl
  derivation), so ecl can figure out the flags to pass on its own.
  Remove NIX_LDFLAGS hack for gmp.

* Make sure ecl can find boehmgc when linking programs / libraries via
  NIX_LDFLAGS. Pass correct -I for boehmgc via NIX_CFLAGS_COMPILE since
  there's no --with-boehmgc-prefix configure flag to achieve the same
  result.

* Use correct suffixSalt for every flag. This makes NIX_CFLAGS_COMPILE
  work properly in the first place and should prevent the extra flags
  hacks leaking into other places in some edge cases.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs