Use env.Append when setting compiler settings

SConstruct

@@ -1,5 +1,4 @@
 import os
-import platform
 
 #version info of the library
 revision = svn_version = os.popen('svnversion').read()[:-1]
@@ -55,18 +54,18 @@ env = Environment(tools = ['default', 'textfile'])
 env.AddMethod(InstallPerm)
 Export('env', 'build_type', 'prefix', 'version_info', 'test_xml')
 
-env['CFLAGS'] = ['-Wall', '-Wextra', '-pipe', '-ggdb']
-env['LINKFLAGS'] = ['-O2']
-env['CPPDEFINES'] = ['_FILE_OFFSET_BITS=64']
+env.Append(CFLAGS = ['-Wall', '-Wextra', '-pipe', '-ggdb'])
+env.Append(LINKFLAGS = ['-O2'])
+env.Append(CPPDEFINES = ['_FILE_OFFSET_BITS=64'])
 
 if build_type == 'release':
-    env['CFLAGS'] += ['-O2']
+    env.Append(CFLAGS = ['-O2'])
 
     #this will turn off assert()
-    env['CPPDEFINES'] += ['NDEBUG']
+    env.Append(CPPDEFINES = ['NDEBUG'])
 
 elif build_type == 'debug':
-    env['CPPDEFINES'] += ['DEBUG']
+    env.Append(CPPDEFINES = ['DEBUG'])
 
 #put all .sconsign files in one place
 env.SConsignFile()