Enable LTO by default on all platforms

Initially, when LTO was first introduced in commit 55a7a94 (Enable LTO and
-march=native, 2019-05-29), it was enabled by default on all platforms. Now
that it is no longer broken in Nix, this old default may be restored.

The reported upstream issue (NixOS/nixpkgs#19098) has
been resolved and the fix is already included in the `nixpkgs-unstable` branch.

This also enables LTO again on platforms other than Darwin, where it was
unintentionally disabled by default due to a programming error.

ext/extconf.rb

@@ -24,12 +24,7 @@
   $CXXFLAGS << ' -march=native -mtune=native'
 end
 
-# darwin nix clang doesn't support lto
-# disable -lto flag for darwin + nix
-# see: https://github.com/sass/sassc-ruby/issues/148
-enable_lto_by_default = (Gem::Platform.local.os == "darwin" && ENV['NIX_CC'].nil?)
-
-if enable_config('lto', enable_lto_by_default)
+if enable_config('lto', true)
   $CFLAGS << ' -flto'
   $CXXFLAGS << ' -flto'
   $LDFLAGS << ' -flto'