Merge staging into closure-size

The most complex problems were from dealing with switches reverted in
the meantime (gcc5, gmp6, ncurses6).
It's likely that darwin is (still) broken nontrivially.

pkgs/stdenv/generic/setup.sh

@@ -450,7 +450,7 @@ substituteAllInPlace() {
 # the environment used for building.
 dumpVars() {
     if [ "$noDumpEnvVars" != 1 ]; then
-        export > "$NIX_BUILD_TOP/env-vars"
+        export > "$NIX_BUILD_TOP/env-vars" || true
     fi
 }