idrisPackages.idris-wrapper: Fix linking to gmp library

This has been broken since 5d18129ce8,
which updated idris from 1.3.0 to 1.3.1, which included
https://github.com/idris-lang/Idris-dev/pull/4472 as the cause of the
error. I'm still not entirely sure why this broke it though.

This now way should be rather future proof, it uses NIX_CFLAGS to pass
gpm link flags to our CC wrapper directly. The
`NIX_CC_WRAPPER_${stdenv.cc.infixSalt}_TARGET_HOST` part I'm pretty sure
is needed for the CC wrapper to know that those CFLAGS are meant for the
cc running on the HOST.

pkgs/development/idris-modules/idris-wrapper.nix

@@ -1,4 +1,4 @@
-{ lib, symlinkJoin, makeWrapper, idris-no-deps, gcc, gmp }:
+{ stdenv, lib, symlinkJoin, makeWrapper, idris-no-deps, gcc, gmp }:
 
 symlinkJoin {
   inherit (idris-no-deps) name src meta;
@@ -7,6 +7,8 @@ symlinkJoin {
   postBuild = ''
     wrapProgram $out/bin/idris \
       --run 'export IDRIS_CC=''${IDRIS_CC:-${lib.getBin gcc}/bin/gcc}' \
-      --suffix LIBRARY_PATH : ${lib.makeLibraryPath [ gmp ]}
+      --set NIX_CC_WRAPPER_${stdenv.cc.infixSalt}_TARGET_HOST 1 \
+      --prefix NIX_CFLAGS_COMPILE " " "-I${lib.getDev gmp}/include" \
+      --prefix NIX_CFLAGS_LINK " " "-L${lib.getLib gmp}/lib"
   '';
 }