xorg.imake: fix cross

```
imake-aarch64-unknown-linux-gnu> checking for cpp... no
imake-aarch64-unknown-linux-gnu> checking if aarch64-unknown-linux-gnu-gcc -E requires -undef... aarch64-unknown-linux-gnu-gcc: fatal error: noinput files
imake-aarch64-unknown-linux-gnu> compilation terminated.
imake-aarch64-unknown-linux-gnu> aarch64-unknown-linux-gnu-gcc: fatal error: no input files
imake-aarch64-unknown-linux-gnu> compilation terminated.
imake-aarch64-unknown-linux-gnu> aarch64-unknown-linux-gnu-gcc: fatal error: no input files
imake-aarch64-unknown-linux-gnu> compilation terminated.
imake-aarch64-unknown-linux-gnu> configure: error: aarch64-unknown-linux-gnu-gcc -E defines unix with or without -undef.  I don't know what to do.
```

it appears that the configure script isn't checking
`${ac_tool_prefix}cpp`

```
for ac_prog in cpp
do
  # Extract the first word of "$ac_prog", so it can be a program name with args.
set dummy $ac_prog; ac_word=$2
{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
$as_echo_n "checking for $ac_word... " >&6; }
if ${ac_cv_path_RAWCPP+:} false; then :
```

1 files changed, 2 insertions, 0 deletions

diff --git a/pkgs/servers/x11/xorg/overrides.nix b/pkgs/servers/x11/xorg/overrides.nix
index baf968fe309fd..a472c19832236 100644
--- a/pkgs/servers/x11/xorg/overrides.nix
+++ b/pkgs/servers/x11/xorg/overrides.nix
@@ -68,6 +68,8 @@ self: super:
       then "${tradcpp}/bin/cpp"
       else "gcc"}\"'";
 
+    configureFlags = attrs.configureFlags or [] ++ [ "ac_cv_path_RAWCPP=${stdenv.cc.targetPrefix}cpp" ];
+
     inherit tradcpp;
   });