add rspec test for overridding construction variables for an individual builer

build_tests/two_sources/build.rb

@@ -0,0 +1,4 @@
+Rscons::Environment.new(echo: :command) do |env|
+  env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
+  env.Program('two_sources', ['one.o', 'two.c'])
+end

build_tests/two_sources/one.c

@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+#ifdef ONE
+int main(int argc, char *argv[])
+{
+    printf("This is a C program with two sources.\n");
+}
+#endif

build_tests/two_sources/two.c

@@ -0,0 +1,3 @@
+#ifdef ONE
+#error ONE should not be defined
+#endif