create Builder instance for each build operation - close #88

build_tests/custom_builder/Rsconscript

@@ -11,7 +11,7 @@ end
 
 build do
   Environment.new do |env|
-    env.add_builder(MySource.new)
+    env.add_builder(MySource)
     env.MySource('inc.h', [])
     env.Program('program.exe', Dir['*.c'])
   end

build_tests/custom_builder/cvar_expansion.rb

@@ -13,7 +13,7 @@ build do
   env = Environment.new do |env|
     env["hdr"] = "inc.h"
     env["src"] = "program.c"
-    env.add_builder(MySource.new)
+    env.add_builder(MySource)
     env.MySource('${hdr}')
     env.Program('program.exe', "${src}")
   end

build_tests/custom_builder/cvar_lambda.rb

@@ -11,7 +11,7 @@ end
 
 build do
   e1 = Environment.new do |env|
-    env.add_builder(MySource.new)
+    env.add_builder(MySource)
     env["one"] = "5"
     env[:cfg] = {val: "9"}
     env["two"] = lambda do |args|

build_tests/custom_builder/multiple_targets.rb

@@ -14,7 +14,7 @@ end
 
 build do
   Environment.new do |env|
-    env.add_builder(CHGen.new)
+    env.add_builder(CHGen)
     env.CHGen("inc.c", ["program.c"])
     env.Program("program.exe", %w[program.c inc.c])
   end