execute all build tests in the top-level rake context instead of spawning subprocesses

build_tests/custom_builder/build.rb

@@ -1,16 +0,0 @@
-class MySource < Rscons::Builder
-  def run(target, sources, cache, env, vars = {})
-    File.open(target, 'w') do |fh|
-      fh.puts <<EOF
-#define THE_VALUE 5678
-EOF
-    end
-    target
-  end
-end
-
-Rscons::Environment.new(echo: :short) do |env|
-  env.add_builder(MySource.new)
-  env.MySource('inc.h', [])
-  env.Program('program', Dir['*.c'])
-end