build: Updated precompiled header support (enabled by default)

Change-Id: Iaab62c740ac87b84256a3f71d934760865a54be5
diff --git a/wscript b/wscript
index c43a556..6c10db5 100644
--- a/wscript
+++ b/wscript
@@ -121,12 +121,12 @@
     core = bld(
         target='core-objects',
         name='core-objects',
-        features='cxx',
+        features='cxx pch',
         source=bld.path.ant_glob(['core/**/*.cpp']),
         use='version BOOST NDN_CXX LIBRT',
         includes='. core',
         export_includes='. core',
-        pch='common-pch.hpp',
+        headers='common.hpp',
         )
 
     nfd_objects = bld(
@@ -141,7 +141,6 @@
         use='core-objects',
         includes='daemon websocketpp',
         export_includes='daemon',
-        pch='common-pch.hpp',
         )
 
     if bld.env['HAVE_LIBPCAP']: