Sort input file list

so that libsass.so builds in a reproducible way
in spite of indeterministic filesystem readdir order.

See https://reproducible-builds.org/ for why this is good.

This PR was done while working on reproducible builds for openSUSE.

ext/extconf.rb

@@ -56,7 +56,7 @@
 $VPATH << "$(srcdir)/libsass/src"
 Dir.chdir(__dir__) do
   $VPATH += Dir['libsass/src/*/'].map { |p| "$(srcdir)/#{p}" }
-  $srcs = Dir['libsass/src/**/*.{c,cpp}']
+  $srcs = Dir['libsass/src/**/*.{c,cpp}'].sort
 end
 
 # Don't link libruby.