treewide: Make still dont* Variables are optional in most cases

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

pkgs/build-support/setup-hooks/auto-patchelf.sh

@@ -228,7 +228,7 @@ autoPatchelf() {
 # behaviour as fixupOutputHooks because the setup hook for patchelf is run in
 # fixupOutput and the postFixup hook runs later.
 postFixupHooks+=('
-    if [ -z "$dontAutoPatchelf" ]; then
+    if [ -z "${dontAutoPatchelf-}" ]; then
         autoPatchelf -- $(for output in $outputs; do
             [ -e "${!output}" ] || continue
             echo "${!output}"

pkgs/build-support/setup-hooks/patch-shebangs.sh

@@ -105,7 +105,7 @@ patchShebangs() {
 }
 
 patchShebangsAuto () {
-    if [ -z "$dontPatchShebangs" -a -e "$prefix" ]; then
+    if [ -z "${dontPatchShebangs-}" -a -e "$prefix" ]; then
 
         # Dev output will end up being run on the build platform. An
         # example case of this is sdl2-config. Otherwise, we can just

pkgs/build-support/setup-hooks/update-autotools-gnu-config-scripts.sh

@@ -1,7 +1,7 @@
 preConfigurePhases+=" updateAutotoolsGnuConfigScriptsPhase"
 
 updateAutotoolsGnuConfigScriptsPhase() {
-    if [ -n "$dontUpdateAutotoolsGnuConfigScripts" ]; then return; fi
+    if [ -n "${dontUpdateAutotoolsGnuConfigScripts-}" ]; then return; fi
 
     for script in config.sub config.guess; do
         for f in $(find . -type f -name "$script"); do

pkgs/build-support/setup-hooks/wrap-gapps-hook.sh

@@ -15,7 +15,7 @@ wrapGApp() {
     wrapProgram "$program" "${gappsWrapperArgs[@]}" "$@"
 }
 
-# Note: $gappsWrapperArgs still gets defined even if $dontWrapGApps is set.
+# Note: $gappsWrapperArgs still gets defined even if ${dontWrapGApps-} is set.
 wrapGAppsHook() {
   # guard against running multiple times (e.g. due to propagation)
   [ -z "$wrapGAppsHookHasRun" ] || return 0