completion/system: accomodate multiple versions

For Homebrew, switch between v1 and v2 of bash-completion based on whether the running Bash shell is new enough to use v2.

completion/available/system.completion.bash

@@ -5,14 +5,16 @@
 
 if shopt -qo nounset
 then # Bash-completion is too large and complex to expect to handle unbound variables throughout the whole codebase.
-	BASH_IT_RESTORE_NOUNSET=true
+	__bash_it_restore_nounset=true
 	shopt -uo nounset
 else
-	BASH_IT_RESTORE_NOUNSET=false
+	__bash_it_restore_nounset=false
 fi
 
 if [[ -r "${BASH_COMPLETION:-}" ]] ; then
+  # shellcheck disable=SC1091
 	source "${BASH_COMPLETION}"
+
 elif [[ -r /etc/bash_completion ]] ; then
   # shellcheck disable=SC1091
   source /etc/bash_completion
@@ -24,15 +26,34 @@ elif [[ -r /etc/profile.d/bash_completion.sh ]] ; then
 
 elif _bash_it_homebrew_check
 then
+  : ${BASH_COMPLETION_COMPAT_DIR:=$BASH_IT_HOMEBREW_PREFIX/etc/bash_completion.d}
+
+  case "${BASH_VERSION}" in
+  1*|2*|3.0*|3.1*)
+    _log_warning "Cannot load completion due to version of shell."
+    ;;
+  3.2*|4.0*|4.1*)
+    # Import version 1.x of bash-completion, if installed.
+    BASH_COMPLETION="$BASH_IT_HOMEBREW_PREFIX/opt/bash-completion@1/etc/bash_completion"
+    if [[ -r "$BASH_COMPLETION" ]] ; then
+      # shellcheck disable=SC1090
+      source "$BASH_COMPLETION"
+    else
+      unset BASH_COMPLETION
+    fi
+    ;;
+  4.2*|5*|*)
     # homebrew/versions/bash-completion2 (required for projects.completion.bash) is installed to this path
     if [[ -r "$BASH_IT_HOMEBREW_PREFIX"/etc/profile.d/bash_completion.sh ]] ; then
       # shellcheck disable=SC1090
       source "$BASH_IT_HOMEBREW_PREFIX"/etc/profile.d/bash_completion.sh
     fi
+    ;;
+  esac
 fi
 
-if $BASH_IT_RESTORE_NOUNSET
+if $__bash_it_restore_nounset
 then
 	shopt -so nounset
 fi
-unset BASH_IT_RESTORE_NOUNSET
+unset __bash_it_restore_nounset