exiv2: clean up
exiv2: clean up

- EXIV2_BUILD_PO has been replaced by EXIV2_ENABLE_NLS
  https://github.com/Exiv2/exiv2/commit/076585d1476283bc7cffcf08e9006fc64ffbfa10
  Confusingly, the former still appears in the flag summary.
- Remove enableParallelBuilding since it is on by default with CMake
- Reorder the expression
- Apply upstream GNUInstallDirs patch
- Build stuff in build phase, not in install phase
- Add maintainers field (empty at the moment)

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