add Environment#build_after

build_tests/custom_builder/build_after.rb

@@ -0,0 +1,8 @@
+Rscons::Environment.new do |env|
+  env.Command("inc.c",
+              [],
+              "CMD" => %w[ruby gen.rb ${_TARGET}],
+              "CMD_DESC" => "Generating")
+  env.build_after("program.o", "inc.c")
+  env.Program("program.exe", ["program.c", "inc.c"])
+end

build_tests/custom_builder/gen.rb

@@ -0,0 +1,9 @@
+sleep 1.0
+c_file = ARGV.first
+h_file = File.basename(c_file, ".c") + ".h"
+File.open(c_file, "w") do |fh|
+  fh.puts
+end
+File.open(h_file, "w") do |fh|
+  fh.puts("#define THE_VALUE 191")
+end