syntax: makefile: color all keywords that GNU make recognizes

Plus one that automake recognizes: if.  Color them only at the start
of a line.  Also color all possible assignment sequences (surrounded
by spaces to not color the ones in shell fragments), and add some
comments.

syntax/makefile.nanorc

@@ -4,11 +4,21 @@ syntax makefile "(/((GNU)?m|M)akefile[^/]*$|\.(make|mk)$)"
 magic "makefile script"
 comment "#"
 
-color red "[:=]"
-color magenta "\<(if|ifeq|else|endif)\>"
+# Assignments.
+color red " (:?:|\+|\?)?= "
+
+# Keywords.
+color magenta "^(if|ifn?def|ifn?eq|else|endif|(-|s)?include)\>"
+color magenta "^((override +)?(un)?define|endef|(un)?export|private|vpath)\>"
+
+# Variable expansions.
 color blue "\$+[{(][a-zA-Z0-9_-]+[})]"
+
+# Targets.
 color brightblue "^[^ 	]+:"
+
+# Comments.
 color green "(^|[[:space:]]+)#.*"
 
-## Trailing whitespace.
+# Trailing whitespace.
 color ,green "[[:space:]]+$"