uqm: Fix build against glibc 2.26

I've bisected the introduction of the build failure to be the glibc 2.26
upgrade with commit 9bb67d5c1e546968fa3d195f0bcad82414243566.

At first I was somewhat stumped why an glibc update could cause
undeclared identifiers, but after looking at the changes of glibc and
the source code of The Ur-Quan Masters the problem quickly turned out to
be this very change:

https://sourceware.org/git/?p=glibc.git;a=commit;h=7b037c095e31c2396d0a9b0e6356bc566ee4812f

So string.h now in turn includes strings.h, which in theory wouldn't be
a problem.

However, both strings.h and the strings.h in the uqm source code use
constant _STRINGS_H, which causes the glibc strings.h to be included but
the one from uqm basically includes an empty file.

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

pkgs/games/uqm/default.nix

@@ -70,6 +70,11 @@ in stdenv.mkDerivation rec {
     ln -s "${videos}" "uqm-${version}/content/addons/3dovideo"
   '';
 
+  # Using _STRINGS_H as include guard conflicts with glibc.
+  postPatch = ''
+    sed -i -e '/^#/s/_STRINGS_H/_UQM_STRINGS_H/g' src/uqm/comm/*/strings.h
+  '';
+
   # uqm has a 'unique' build system with a root script incidentally called
   # 'build.sh'.
   configurePhase = ''