Rework builder interface to only use #run method - close #91

The builder's #run method will be called repeatedly until it returns
true or false. The Builder#wait_for method can be used to cause a
builder to wait for a Thread, Command, or another Builder.

build_tests/custom_builder/Rsconscript

@@ -5,7 +5,7 @@ class MySource < Rscons::Builder
 #define THE_VALUE 5678
 EOF
     end
-    @target
+    true
   end
 end
 

build_tests/custom_builder/cvar_expansion.rb

@@ -5,7 +5,7 @@ class MySource < Rscons::Builder
 #define THE_VALUE 678
 EOF
     end
-    @target
+    true
   end
 end
 

build_tests/custom_builder/cvar_lambda.rb

@@ -5,7 +5,7 @@ class MySource < Rscons::Builder
 #define THE_VALUE #{@env.expand_varref("${the_value}")}
 EOF
     end
-    @target
+    true
   end
 end
 

build_tests/custom_builder/error_run_return_value.rb

@@ -0,0 +1,8 @@
+build do
+  Environment.new do |env|
+    env.add_builder(:MyBuilder) do |options|
+      "hi"
+    end
+    env.MyBuilder("foo")
+  end
+end

build_tests/custom_builder/error_wait_for.rb

@@ -0,0 +1,8 @@
+build do
+  Environment.new do |env|
+    env.add_builder(:MyBuilder) do |options|
+      wait_for(1)
+    end
+    env.MyBuilder("foo")
+  end
+end

build_tests/custom_builder/multiple_targets.rb

@@ -8,7 +8,7 @@ class CHGen < Rscons::Builder
       File.open(h_fname, "w") {|fh| fh.puts "extern int THE_VALUE;"}
       @cache.register_build([c_fname, h_fname], "", @sources, @env)
     end
-    @target
+    true
   end
 end
 

build_tests/custom_builder/wait_for_thread.rb

@@ -0,0 +1,21 @@
+class MyBuilder < Rscons::Builder
+  def run(options)
+    if @thread
+      true
+    else
+      @env.print_builder_run_message("#{name} #{target}", nil)
+      @thread = Thread.new do
+        sleep 2
+        FileUtils.touch(@target)
+      end
+      wait_for(@thread)
+    end
+  end
+end
+
+build do
+  Environment.new do |env|
+    env.add_builder(MyBuilder)
+    env.MyBuilder("foo")
+  end
+end