enhance makefile completion to support multiple files and gnumake

completion/available/makefile.completion.bash

@@ -1,3 +1,24 @@
-# Add completion for Makefile
-# see http://stackoverflow.com/a/38415982/1472048
-complete -W "\`grep -oE '^[a-zA-Z0-9_-]+:([^=]|$)' Makefile | sed 's/[^a-zA-Z0-9_-]*$//'\`" make
+#!/usr/bin/env bash
+# Bash completion for Makefile
+
+# Loosely adapted from http://stackoverflow.com/a/38415982/1472048
+
+_makecomplete() {
+  # https://www.gnu.org/software/make/manual/html_node/Makefile-Names.html
+  local files=( $(find . -maxdepth 1 -regextype posix-extended -regex '.*(GNU)?[Mm]akefile$' -printf '%f ') )
+
+  # collect all targets
+  local targets=''
+  for f in ${files[@]} ; do
+    for t in $(grep -oE '^[a-zA-Z0-9_-]+:([^=]|$)' $f | cut -d':' -f1) ; do
+      targets+="$t\n"
+    done
+  done
+
+  # flatten the array for completion
+  COMPREPLY=($(compgen -W "$(echo -e "$targets" | head -c -1 | sort -u)" -- ${COMP_WORDS[COMP_CWORD]}))
+  return 0
+}
+
+complete -o nospace -F _makecomplete make
+complete -o nospace -F _makecomplete gnumake