vim: Harden build against ex and other tools being missing

Those tools are often used as build-time dependency. Some deficiencies
in the upstream makefiles makes it possible that they will not be
installed, even though the `make install` call was successful.

This *could* make the `vim` build fail where it wouldn't have failed
previously, but it will now fail in a correct manner instead of being
incorrectly successful.

The previous patch (actually using -j1 in install phase) should make
this change redundant, but let's not pretend new bugs are never going to
happen!

pkgs/applications/editors/vim/default.nix

@@ -50,6 +50,15 @@ stdenv.mkDerivation {
     ln -s $out/bin/vim $out/bin/vi
     mkdir -p $out/share/vim
     cp "${vimrc}" $out/share/vim/vimrc
+
+    # Prevent bugs in the upstream makefile from silently failing and missing outputs.
+    # Some of those are build-time requirements for other packages.
+    for tool in ex xxd vi view vimdiff; do
+      if [ ! -e "$out/bin/$tool" ]; then
+        echo "ERROR: install phase did not install '$tool'."
+        exit 1
+      fi
+    done
   '';
 
   __impureHostDeps = [ "/dev/ptmx" ];