uwimap: Fixed cross-compilation

Shouldn't cause a rebuild of native package. The build system, as with
Lua, couldn't find cc, ar and ranlib - I helped it a little.

pkgs/tools/networking/uwimap/default.nix

@@ -1,6 +1,6 @@
 {stdenv, fetchurl, pam, openssl}:
 
-stdenv.mkDerivation {
+stdenv.mkDerivation ({
   name = "uw-imap-2007f";
 
   src = fetchurl {
@@ -12,7 +12,8 @@ stdenv.mkDerivation {
     then "osx"
     else "lnp" # Linux with PAM modules;
     # -fPIC is required to compile php with imap on x86_64 systems
-    + stdenv.lib.optionalString stdenv.isx86_64 " EXTRACFLAGS=-fPIC";
+    + stdenv.lib.optionalString stdenv.isx86_64 " EXTRACFLAGS=-fPIC"
+    + stdenv.lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) " CC=${stdenv.hostPlatform.config}-gcc RANLIB=${stdenv.hostPlatform.config}-ranlib";
 
   hardeningDisable = [ "format" ];
 
@@ -46,4 +47,11 @@ stdenv.mkDerivation {
   passthru = {
     withSSL = true;
   };
-}
+} // stdenv.lib.optionalAttrs (stdenv.buildPlatform != stdenv.hostPlatform) {
+  # This is set here to prevent rebuilds on native compilation.
+  # Configure phase is a no-op there, because this package doesn't use ./configure scripts.
+  configurePhase = ''
+    echo "Cross-compilation, injecting make flags"
+    makeFlagsArray+=("ARRC=${stdenv.hostPlatform.config}-ar rc")
+  '';
+})