improve extconf so it compiles when called from the project root dir

ext/extconf.rb

@@ -1,18 +1,34 @@
+require 'rbconfig'
+
 # Workaround to make Rubygems believe it builds a native gem
-require 'mkmf'
-create_makefile('none')
+File.open(File.expand_path('../Makefile', __FILE__), 'w') do |f|
+  f.puts("install:\n\t$(exit 0)")
+end
 
-if `uname -s`.chomp == 'Darwin'
-  gem_root      = File.expand_path(File.join('..'))
+if Config::CONFIG['host_os'].to_s =~ /\b(darwin|osx)/i
   darwin_verion = `uname -r`.to_i
   sdk_verion    = { 9 => '10.5', 10 => '10.6', 11 => '10.7' }[darwin_verion]
 
   raise "Darwin #{darwin_verion} is not supported" unless sdk_verion
 
-  # Compile the actual fsevent_watch binary
-  system("CFLAGS='-isysroot /Developer/SDKs/MacOSX#{sdk_verion}.sdk -mmacosx-version-min=#{sdk_verion}' /usr/bin/gcc -framework CoreServices -o '#{gem_root}/bin/fsevent_watch' fsevent/fsevent_watch.c")
+  source_file = File.expand_path("../fsevent/fsevent_watch.c", __FILE__)
+  target_bin  = File.expand_path("../../bin/#{File.basename(source_file, '.c')}", __FILE__)
+
+  old_cflags  = ENV['CFLAGS']
+
+  cflags = %w[-isysroot] <<
+    "/Developer/SDKs/MacOSX#{sdk_verion}.sdk" <<
+    "-mmacosx-version-min=#{sdk_verion}" <<
+    old_cflags
+
+  ENV['CFLAGS'] = cflags.join(' ')
 
-  unless File.executable?("#{gem_root}/bin/fsevent_watch")
-    raise "Compilation of fsevent_watch failed (see README)"
+  begin
+    # Compile the actual fsevent_watch binary
+    system('gcc', '-framework', 'CoreServices', '-o', target_bin, source_file)
+  ensure
+    ENV['CFLAGS'] = old_cflags
   end
+
+  raise 'Compiling "fsevent_watch" failed' unless File.executable? target_bin
 end