exiv2: clean up

[jtojnar]

Motivation for this change

• EXIV2_BUILD_PO has been replaced by EXIV2_ENABLE_NLS
  Exiv2/exiv2@076585d
  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 (cmake: Fix paths with absolute GNUInstallDirs Exiv2/exiv2#1275)
• Build stuff in build phase, not in install phase
• Add maintainers field (empty at the moment)

cc @paperdigits

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[timokau]

It looks like this didn't quite work as intended, it broke the digikam build:

--  ==> Graphviz Dot is found:     NO
-- Configuring done
CMake Error in core/app/CMakeLists.txt:
  Imported target "exiv2lib" includes non-existent path

    "/nix/store/qx8azn7x87g6w56j14cvhykfxqwv1vmf-exiv2-0.27.3/include"

  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:

  * The path was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and references files it does not
  provide.

Apparently exiv still pretends that its include directory is in the out output. I don't know much about cmake, but it looks like this is specified in exiv2.dev/lib/cmake/exiv2/exiv2Config.cmake. Any idea why this is happening?