Do not rebuild after building after auto-configuration. - close #80

Split Environment into BasicEnvironment and Environment.

build_tests/configure/autoconf_rebuild.rb

@@ -0,0 +1,10 @@
+configure do
+  check_c_compiler
+  check_c_header "stdio.h"
+end
+
+build do
+  Environment.new do |env|
+    env.Program("simple.exe", "simple.c")
+  end
+end