1st chuck is for scons 4
3rd chunk is needed for proper boost_thread detection

Index: SConstruct
--- SConstruct.orig
+++ SConstruct
@@ -126,7 +126,6 @@ PLUGINS = { # plugins with external dependencies
 
 def init_environment(env):
     env.Decider('MD5-timestamp')
-    env.SourceCode(".", None)
     if os.environ.get('RANLIB'):
         env['RANLIB'] = os.environ['RANLIB']
     if os.environ.get('AR'):
@@ -297,7 +296,7 @@ opts.AddVariables(
     ('CUSTOM_LDFLAGS', 'Custom linker flags, e.g. -L<lib dir> if you have libraries in a nonstandard directory <lib dir>', ''),
     EnumVariable('LINKING', "Set library format for libmapnik",'shared', ['shared','static']),
     EnumVariable('RUNTIME_LINK', "Set preference for linking dependencies",'shared', ['shared','static']),
-    EnumVariable('OPTIMIZATION','Set compiler optimization level','3', ['0','1','2','3','4','s']),
+    EnumVariable('OPTIMIZATION','Set compiler optimization level','2', ['0','1','2','3','4','s']),
     # Note: setting DEBUG=True will override any custom OPTIMIZATION level
     BoolVariable('DEBUG', 'Compile a debug version of Mapnik', 'False'),
     BoolVariable('COVERAGE', 'Compile a libmapnik and plugins with --coverage', 'False'),
@@ -1364,6 +1363,9 @@ if not preconfigured:
 
     thread_suffix = 'mt'
     if env['PLATFORM'] == 'FreeBSD':
+        thread_suffix = ''
+        env.Append(LIBS = 'pthread')
+    if env['PLATFORM'] == 'OpenBSD':
         thread_suffix = ''
         env.Append(LIBS = 'pthread')