nokogumbo-import: get cross-compiling working on windows

Note that we're using a new feature in mini_portile 2.5.1 to pass
environment variables to `#execute` for the subprocess.

This allows us to pass a single command string to `Process.spawn` and
take advantage of the subshell's command substitution (e.g.,
rake-compiler's "MAKE=nice make -j`nproc`").

This also allows us to have environment variables with embedded spaces
without worrying about cross-platform escaping (e.g., "CFLAGS=-fPIC -g").

ext/nokogiri/extconf.rb

@@ -897,8 +897,10 @@ def install
 
     def compile
       cflags = concat_flags(ENV["CFLAGS"], "-fPIC", "-g")
-      command = [make_cmd, "CC=#{gcc_cmd}", "CFLAGS=#{cflags}"]
-      execute("compile", command)
+
+      env = {"CC" => gcc_cmd, "CFLAGS" => cflags}
+
+      execute("compile", make_cmd, {env: env})
     end
   end
 end