set Environment build root automatically - close #64

build_tests/simple/absolute_source_path.rb

@@ -1,5 +1,4 @@
 Rscons::Environment.new do |env|
-  env.build_root = "build"
   tempdir = ENV["TEMP"] || ENV["TMP"] || "/tmp"
   source_file = File.join(tempdir, "abs.c")
   File.open(source_file, "w") do |fh|

build_tests/simple/builder_no_sources.rb

@@ -4,7 +4,6 @@ class TestBuilder < Rscons::Builder
   end
 end
 Rscons::Environment.new do |env|
-  env.build_root = "build"
   env.add_builder(TestBuilder.new)
   env.TestBuilder("file")
 end