enc: Expand substitution variables in Makefile.in by default

This change makes `make_encmake.rb` expand all substitution variables
when replacing them in `enc/Makefile.in` to avoid propagating all
possibly referenced variables to the generated Makefile. The old
behavior, which don't expand make variables when generating Makefile,
was useful to temporarily override inherited variables like `cflags` in
`CFLAGS` at make-time. However, it's not a common use case and it
requires to propagate all possibly referenced variables properly
considering key name duplication between `enc/Makefile.in` and
`RbConfig::CONFIG`.

enc/make_encmake.rb

@@ -129,7 +129,12 @@ def target_transcoders
   dep = ""
 end
 mkin = File.read(File.join($srcdir, "Makefile.in"))
-mkin.gsub!(/@(#{CONFIG.keys.join('|')})@/) {CONFIG[$1]}
+# Variables that should not be expanded in Makefile.in to allow
+# overriding inherited variables at make-time.
+not_expand_vars = %w(CFLAGS)
+mkin.gsub!(/@(#{RbConfig::CONFIG.keys.join('|')})@/) do
+  not_expand_vars.include?($1) ? CONFIG[$1] : RbConfig::CONFIG[$1]
+end
 File.open(ARGV[0], 'wb') {|f|
   f.puts mkin, dep
 }