add build test using a build directory

build_tests/build_dir/build.rb

@@ -0,0 +1,5 @@
+Rscons::Environment.new do |env|
+  env.append('CPPPATH' => Dir['src/**/*/'])
+  env.build_dir('src', 'build')
+  env.Program('build_dir', Dir['src/**/*.c'])
+end

build_tests/build_dir/src/one/one.c

@@ -0,0 +1,6 @@
+#include "two.h"
+
+int main(int argc, char *argv[])
+{
+    two();
+}

build_tests/build_dir/src/two/two.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+#include "two.h"
+
+void two(void)
+{
+    printf("Hello from two()\n");
+}

build_tests/build_dir/src/two/two.h

@@ -0,0 +1,6 @@
+#ifndef TWO_H
+#define TWO_H
+
+void two(void);
+
+#endif