add build test for custom builders

build_tests/custom_builder/build.rb

@@ -0,0 +1,16 @@
+class MySource < Rscons::Builder
+  def run(target, sources, cache)
+    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))
+  env.MySource('inc.h', [])
+  env.Program('program', Dir['*.c'])
+end

build_tests/custom_builder/program.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+#include "inc.h"
+
+int main(int argc, char *argv[])
+{
+    printf("The value is %d\n", THE_VALUE);
+}