uqm: Fix purity checking error during config phase

Since commit fb777be7d2, linking programs
in /tmp will no longer work, since with this commit (hardcoded) /tmp
will no longer be treated a valid temporary directory by the purity
checking logic.

Essentially what that logic does is checking whether the link target is
either relative, in some temporary directory or in the Nix store and if
that's not the case, an error is thrown like in this case:

  gcc -c /tmp/build.24.tmp.c -o /tmp/build.24.tmp.c.o
  gcc /tmp/build.24.tmp.c.o -o /tmp/build.24.tmp.out
  impure path `/tmp/build.24.tmp.out' used in link
  collect2: error: ld returned 1 exit status

Note here, that $TMP and $TMPDIR point to /build while the output here
still refers to /tmp. This is because the custom build system of uqm
simply hardcodes /tmp and doesn't honor any of these variables.

To fix this, all we need to do is the latter and we're now using $TMPDIR
instead of /tmp for these compile-tests during configure.

While the pull request[1] introducing said commit has a few comments
regarding other issues and it could still be either reverted or changed,
using $TMPDIR in the uqm build script is entirely backwards-compatible
and shouldn't hurt in any case.

[1]: https://github.com/NixOS/nixpkgs/pull/93560

Signed-off-by: aszlig <aszlig@nix.build>

pkgs/games/uqm/default.nix

@@ -65,9 +65,11 @@ in stdenv.mkDerivation rec {
     ln -s "${videos}" "uqm-${version}/content/addons/3dovideo"
   '';
 
-  # Using _STRINGS_H as include guard conflicts with glibc.
   postPatch = ''
+    # Using _STRINGS_H as include guard conflicts with glibc.
     sed -i -e '/^#/s/_STRINGS_H/_UQM_STRINGS_H/g' src/uqm/comm/*/strings.h
+    # See https://github.com/NixOS/nixpkgs/pull/93560
+    sed -i -e 's,/tmp/,$TMPDIR/,' build/unix/config_functions
   '';
 
   # uqm has a 'unique' build system with a root script incidentally called