add build test for custom builders

build_tests/custom_builder/build.rb

@@ -0,0 +1,16 @@
+class MySource < Rscons::Builder
+  def run(target, sources, cache)
+    File.open(target, 'w') do |fh|
+      fh.puts <<EOF
+#define THE_VALUE 5678
+EOF
+    end
+    target
+  end
+end
+
+Rscons::Environment.new(echo: :short) do |env|
+  env.add_builder(MySource.new(env))
+  env.MySource('inc.h', [])
+  env.Program('program', Dir['*.c'])
+end

build_tests/custom_builder/program.c

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

lib/rscons/environment.rb

@@ -88,7 +88,10 @@ module Rscons
       end
       @targets.each do |target, info|
         next if targets_processed.include?(target)
-        break unless process_target.call(target)
+        unless process_target.call(target)
+          $stderr.puts "Error: failed to build #{target}"
+          break
+        end
       end
       cache.write
     end

spec/build_tests_spec.rb

@@ -119,4 +119,11 @@ describe Rscons do
     File.exists?('build/one/one.o').should be_true
     File.exists?('build/two/two.o').should be_true
   end
+
+  it 'allows Ruby classes as custom builders to be used to construct files' do
+    lines = test_dir('custom_builder')
+    lines.should == ['CC program.o', 'LD program']
+    File.exists?('inc.h').should be_true
+    `./program`.should == "The value is 5678\n"
+  end
 end