fill out c-simple build test

build_tests/c-simple/simple.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+    printf("This is a simple C program\n");
+}

build_tests/c-simple/spec.rb

@@ -1,2 +1,5 @@
 it 'builds a C program with one source file' do
+  env = Rscons::Environment.new
+  env.Program('simple', 'simple.c')
+  `./simple`.should =~ /This is a simple C program/
 end