jam: set std=c89

jam relies on c89 conventions; this restores the ability to build on recent Darwin.

pkgs/development/tools/build-managers/jam/default.nix

···
     depsBuildBuild = [ buildPackages.stdenv.cc ];
     nativeBuildInputs = [ bison ];
 
+    # Jam uses c89 conventions
+    env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-std=c89";
+
     # Jambase expects ar to have flags.
     preConfigure = ''
       export AR="$AR rc"