treewide: Fix unsafe concatenation of $LD_LIBRARY_PATH

Naive concatenation of $LD_LIBRARY_PATH can result in an empty
colon-delimited segment; this tells glibc to load libraries from the
current directory, which is definitely wrong, and may be a security
vulnerability if the current directory is untrusted.  (See #67234, for
example.)  Fix this throughout the tree.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pkgs/development/compilers/mlton/20130715.nix

@@ -77,7 +77,7 @@ stdenv.mkDerivation rec {
     chmod u+x $(pwd)/../${usr_prefix}/bin/mlton
 
     # So the builder runs the binary compiler with gmp.
-    export LD_LIBRARY_PATH=${gmp.out}/lib:$LD_LIBRARY_PATH
+    export LD_LIBRARY_PATH=${gmp.out}/lib''${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH
 
   '' + stdenv.lib.optionalString stdenv.isLinux ''
     # Patch ELF interpreter.