srt: Fix wrong srt.pc include path

[nh2]

Motivation for this change

Fixes #70872.

Fix copied from:

nixpkgs/pkgs/development/compilers/hhvm/default.nix

Lines 41 to 44 in 1230fc8

	# the cmake package does not handle absolute CMAKE_INSTALL_INCLUDEDIR correctly
	# (setting it to an absolute path causes include files to go to $out/$out/include,
	# because the absolute path is interpreted with root at $out).
	cmakeFlags = "-DCMAKE_INSTALL_INCLUDEDIR=include";

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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.

Notify maintainers

cc @nh2

FYI @j0sh

[nh2]

After merging this should be backported to release-19.09.

[j0sh]

Thanks for this! Can confirm with a quick test [1] that this fix works.

[1] Overriding the cmakeFlags attribute to insert the new flag:

let
  nixpkgs = import <nixpkgs> {};
  srt = nixpkgs.srt.overrideAttrs (oldAttrs: rec {  cmakeFlags = nixpkgs.srt.cmakeFlags ++ [ "-DCMAKE_INSTALL_INCLUDEDIR=include" ]; }) ;
in
  with nixpkgs;
  stdenv.mkDerivation rec {
    name = "SRT_Fixed";
    env = buildEnv  { name = name; paths = buildInputs; };
    buildInputs = [ pkg-config srt ];
  }

Results in:

$ pkg-config --cflags --libs srt
-I/nix/store/82dkq0hb40ifgqm0kdg2mwkbpnsgjmip-srt-1.3.2/include -I/nix/store/82dkq0hb40ifgqm0kdg2mwkbpnsgjmip-srt-1.3.2/include/srt -L/nix/store/82dkq0hb40ifgqm0kdg2mwkbpnsgjmip-srt-1.3.2/lib -lsrt

[nh2]

Linking issue that collects such CMake path issues: #144170