arj: fix build on darwin

Fix wrong header for statfs function.

Use gccStdenv to build with GCC on all systems. GCC is hardcoded in
configure.in, but after setting CC_FOR_BUILD to cc to use Clang, the
build fails anyway when running a post-compilation command:

    clang -DLOCALE=LANG_en -DLOCALE_DESC="\"en\"" -DPKGLIBDIR="\"<out>/lib/arj\"" -D_UNIX  -g -O2   -o darwin20.6.0/en/rs/arj/arj <objects_files>
    : darwin20.6.0/en/rs/arj/arj
    ./darwin20.6.0/en/rs/tools/join ./darwin20.6.0/en/rs/arj/arj ./darwin20.6.0/en/rs/arjsfxjr/arjsfxjr
    JOIN v 1.30  [26/04/2003]  Not a part of any binary package!

    Copying .. done!
    ./darwin20.6.0/en/rs/tools/join ./darwin20.6.0/en/rs/arj/arj ./darwin20.6.0/en/rs/arjsfx/arjsfx
    JOIN v 1.30  [26/04/2003]  Not a part of any binary package!

    Copying ... done!
    ./darwin20.6.0/en/rs/tools/join ./darwin20.6.0/en/rs/arj/arj ./darwin20.6.0/en/rs/arjsfxv/arjsfxv
    JOIN v 1.30  [26/04/2003]  Not a part of any binary package!

    Copying ..... done!
    ./darwin20.6.0/en/rs/tools/join ./darwin20.6.0/en/rs/arj/arj ./darwin20.6.0/en/rs/sfxstub/sfxstub
    JOIN v 1.30  [26/04/2003]  Not a part of any binary package!

    Copying . done!
    rm -f ./darwin20.6.0/en/rs/help.arj
    TZ=UTC0 ./darwin20.6.0/en/rs/arj/arj a ./darwin20.6.0/en/rs/help.arj -+ -t1f -2d -e -jm -jh65535 -jt -hdo200506231314 ./resource/en/arj?.txt
    ARJ32 v 3.10, Copyright (c) 1998-2004, ARJ Software Russia.

    Creating archive  : ./darwin20.6.0/en/rs/help.arj
    Adding    ./resource/en/arjl.txt       37.2%
    Adding    ./resource/en/arjs.txt       47.3%
    Testing arjl.txt                    OK
    Testing arjs.txt                    OK
         2 file(s)
    ./darwin20.6.0/en/rs/tools/join ./darwin20.6.0/en/rs/arj/arj ./darwin20.6.0/en/rs/help.arj
    JOIN v 1.30  [26/04/2003]  Not a part of any binary package!

    Copying . done!
    ./darwin20.6.0/en/rs/tools/postproc darwin20.6.0/en/rs/arj/arj
    POSTPROC v 1.30  [17/01/2003]  Not a part of any binary package!

    Patch not found
    make[1]: *** [GNUmakefile:398: darwin20.6.0/en/rs/arj/arj] Error 3

pkgs/tools/archivers/arj/default.nix

@@ -128,6 +128,11 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ autoreconfHook ];
 
+  postPatch = lib.optionalString stdenv.isDarwin ''
+    substituteInPlace environ.c \
+        --replace "  #include <sys/statfs.h>" "  #include <sys/mount.h>"
+  '';
+
   preAutoreconf = ''
     cd gnu
   '';

pkgs/top-level/all-packages.nix

@@ -1281,7 +1281,9 @@ with pkgs;
 
   argyllcms = callPackage ../tools/graphics/argyllcms {};
 
-  arj = callPackage ../tools/archivers/arj { };
+  arj = callPackage ../tools/archivers/arj {
+    stdenv = gccStdenv;
+  };
 
   arp-scan = callPackage ../tools/misc/arp-scan { };