Make scons respect user variables

--- a/setup.py
+++ b/setup.py
@@ -214,7 +214,7 @@
 
 # The following variables are used to customize the compile process
 
-INC_DIRS = []
+INC_DIRS = ['/usr/include']
 LIB_DIRS = []
 LIBS     = []
 CC_ARGS  = []
--- a/supportlib/SConstruct
+++ b/supportlib/SConstruct
@@ -30,19 +30,11 @@
 # Add the local 'include' directory...
 env.Append(CPPPATH = ["include"])
 
-# Do platform specific stuff...
-if sys.platform=="win32":
-    env.Append(CCFLAGS = ["/GX", "/GR", "/MD", "/W3"])
-    env.Append(CPPDEFINES = ["WIN32", "_LIB"])
-elif sys.platform=="darwin":
-    env.Append(CCFLAGS = ["-arch", "x86_64"])
-    env.Append(CCFLAGS = ["-arch", "i386"])
-#    env.Append(CCFLAGS = ["-arch", "ppc"])
-    env.Append(CPPPATH = ["/opt/local/include"])
-    env.Append(CCFLAGS = ["-fPIC"])
-else:
-    env.Append(CPPPATH = ["/opt/local/include"])
-    env.Append(CCFLAGS = ["-fPIC"])
+env.Append(CPPPATH = ['/usr/include'])
+env['AR'] = os.environ.get('AR', 'ar')
+env['CXX'] = os.environ.get('CXX', 'g++')
+env['CXXFLAGS'] = os.environ.get('CXXFLAGS', '-fPIC')
+env['CPPFLAGS'] = os.environ.get('CPPFLAGS', '')
 
 # Setup the help message
 Help(vars.GenerateHelpText(env))