add spec for building with a header file

spec/build_items/header.c

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

spec/build_items/header.h

@@ -0,0 +1,6 @@
+#ifndef HEADER_H
+#define HEADER_H
+
+#define VALUE 33
+
+#endif

spec/build_tests_spec.rb

@@ -72,4 +72,13 @@ describe Rscons do
       `./simple`.should =~ /This is a modified C program/
     end
   end
+
+  it 'builds a C program with a header file' do
+    setup_testdir(['header.c', 'header.h']) do
+      env = Rscons::Environment.new
+      env.Program('header', 'header.c')
+      File.exist?('header.o').should be_true
+      `./header`.should =~ /The value is 33/
+    end
+  end
 end