--- a/SConstruct
+++ b/SConstruct
@@ -317,7 +317,7 @@
     ('LINK_PRIORITY','Priority list in which to sort library and include paths (default order is internal, other, frameworks, user, then system - see source of `sort_paths` function for more detail)',','.join(DEFAULT_LINK_PRIORITY)),
 
     # Install Variables
-    ('PREFIX', 'The install path "prefix"', '/usr/local'),
+    ('PREFIX', 'The install path "prefix"', '/usr'),
     ('LIBDIR_SCHEMA', 'The library sub-directory appended to the "prefix", sometimes lib64 on 64bit linux systems', LIBDIR_SCHEMA_DEFAULT),
     ('DESTDIR', 'The root directory to install into. Useful mainly for binary package building', '/'),
     ('PATH', 'A custom path (or multiple paths divided by ":") to append to the $PATH env to prioritize usage of command line programs (if multiple are present on the system)', ''),
@@ -1222,6 +1222,8 @@
 
     color_print(4,'Configuring build environment...')
 
+    OPTIONAL_LIBSHEADERS = []
+
     if not env['FAST']:
         SetCacheMode('force')
 
@@ -1410,7 +1412,8 @@
     # https://github.com/mapnik/mapnik/issues/913
     if env.get('XMLPARSER') and env['XMLPARSER'] == 'libxml2':
         if env.get('XML2_LIBS') or env.get('XML2_INCLUDES'):
-            OPTIONAL_LIBSHEADERS.insert(0,['libxml2','libxml/parser.h',True,'C'])
+            # code doesn't actually use HAVE_XMML2 but set just to have a fifth value
+            OPTIONAL_LIBSHEADERS.append(['libxml2','libxml/parser.h',True,'C','-DHAVE_XML2'])
             if env.get('XML2_INCLUDES'):
                 inc_path = env['XML2_INCLUDES']
                 env.AppendUnique(CPPPATH = fix_path(inc_path))
@@ -1428,8 +1431,6 @@
         else:
             env['SKIPPED_DEPS'].extend(['dlfcn'])
 
-    OPTIONAL_LIBSHEADERS = []
-
     if env['JPEG']:
         OPTIONAL_LIBSHEADERS.append(['jpeg', ['stdio.h', 'jpeglib.h'], False,'C','-DHAVE_JPEG'])
         inc_path = env['%s_INCLUDES' % 'JPEG']