libsigc++ pkg-config file forces -std=c++11

[jeroendehaas]

Issue description

Currently, the derivation for libsigc++ includes a patch that appends -std=c++11 to sigc++-2.0.pc. As a result a C++ project that uses libsigc++-2.0 may be interpreted according to a different standard than expected.

For instance, GCC 6.1 and higher by default interprets C++ code according to the C++14 standard and using libsigc++ via pkg-config effectively downgrades this to C++11.

The patch responsible seems to originate from the Debian project where it was introduced in 2015 when GCC 5 was their default compiler and removed in 2016 when they switched to GCC 6.

As a side note, I feel pkg-config files are not the place to force a certain c++ standard.

Steps to reproduce

1. Create a c++ file that conforms to C++14 and higher (but not lower)

decltype(auto) cxx14() {
    return 0;
}
int main() {
    return cxx14();
}

2. Create a nix expression, e.g. default.nix that lists gcc, pkgconfig and libsigcxx as buildInputs

with import <nixpkgs> {} ;
stdenv.mkDerivation rec {
    src = ./.;
    name = "repr";
    buildInputs = [ gcc pkgconfig libsigcxx];
}

3. Enter nix-shell
4. Run the following command

gcc `pkg-config --cflags --libs sigc++-2.0` file.cpp

Technical details

% nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 4.14.12, NixOS, 17.09.2631.0653b73bf61 (Hummingbird)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.16`
 - channels(root): `"nixos-17.09.2631.0653b73bf61"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`