codec2: fix cross compilation

It uses cmake flags and environment variables in a way that interacts
poorly with stdenv's defaults. See code comments for details.

pkgs/by-name/co/codec2/package.nix

@@ -1,6 +1,7 @@
 {
   lib,
   stdenv,
+  buildPackages,
   fetchFromGitHub,
   cmake,
   freedvSupport ? false,
@@ -18,12 +19,23 @@ stdenv.mkDerivation rec {
     hash = "sha256-69Mp4o3MgV98Fqfai4txv5jQw2WpoPuoWcwHsNAFPQM=";
   };
 
-  nativeBuildInputs = [ cmake ];
+  nativeBuildInputs = [
+    cmake
+    buildPackages.stdenv.cc # needs to build a C program to run at build time
+  ];
 
   buildInputs = lib.optionals freedvSupport [
     lpcnetfreedv
   ];
 
+  # we need to unset these variables from stdenv here and then set their equivalents in the cmake flags
+  # otherwise it will pass the same compiler to the native and cross phases and crash trying to execute
+  # host binaries (generate_codebook) on the build system.
+  preConfigure = ''
+    unset CC
+    unset CXX
+  '';
+
   postInstall = ''
     install -Dm0755 src/{c2enc,c2sim,freedv_rx,freedv_tx,cohpsk_*,fdmdv_*,fsk_*,ldpc_*,ofdm_*} -t $out/bin/
   '';
@@ -37,6 +49,8 @@ stdenv.mkDerivation rec {
     [
       # RPATH of binary /nix/store/.../bin/freedv_rx contains a forbidden reference to /build/
       "-DCMAKE_SKIP_BUILD_RPATH=ON"
+      "-DCMAKE_C_COMPILER=${stdenv.cc.targetPrefix}cc"
+      "-DCMAKE_CXX_COMPILER=${stdenv.cc.targetPrefix}c++"
     ]
     ++ lib.optionals freedvSupport [
       "-DLPCNET=ON"