redstore: fix build under recent glibc

Previously the build would fail due to, e.g., `getaddrinfo` not being
defined.

(cherry picked from commit a4ae7b87f0e79960615fc77bd80fb5cadbf4f22c)

pkgs/servers/http/redstore/default.nix

···
   };
 
   buildInputs = [ gmp pkgconfig redland zlib librdf_raptor2 librdf_rasqal ];
-     
+
   preConfigure = ''
-    export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -I${librdf_raptor2}/include/raptor2"
-    export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -I${librdf_rasqal}/include/rasqal"
+    # Define _XOPEN_SOURCE to enable, e.g., getaddrinfo.
+    configureFlagsArray+=(
+      "CFLAGS=-D_XOPEN_SOURCE=600 -I${librdf_raptor2}/include/raptor2 -I${librdf_rasqal}/include/rasqal"
+    )
   '';
 
   meta = {