Bump lxml limit and re-enable Append example [skip appveyor]

In the initial version of PR 4235, an added example was left
commented out because it triggered an lxml failure. Thanks to
@jcbrill, reminded that there's a global limit for lxml,
which SCons can set in the docbook module. This is being
bumped a bit, allowing the build to work.

Rebased at the same time, so force-pushing.

Signed-off-by: Mats Wichmann <mats@linux.com>

SCons/Environment.xml

@@ -620,21 +620,22 @@ specify the value as <constant>None</constant>.
 </para>
 
 <!--TODO: at last check, this newly created example blew up SConsDoc/lxml -->
-<!--example_commands>
+<example_commands>
 env = Environment()
 env.Append(CPPDEFINES =[("ONE", 1), ("TWO", )])
 print("CPPDEFINES =", env['CPPDEFINES'])
 env.Append(CPPDEFINES ={"THREE": 3, "FOUR": None})
 print("CPPDEFINES =", env['CPPDEFINES'])
 print("CPPDEFINES will expand to ", env.subst('$_CPPDEFFLAGS'))
-</example_commands-->
+</example_commands>
 
-<!--screen>
+<screen>
+$ scons -Q
 CPPDEFINES = [('ONE', 1), ('TWO',)]
 CPPDEFINES = [('ONE', 1), ('TWO',), {'THREE': 3, 'FOUR': None}]
 CPPDEFINES will expand to  -DONE=1 -DTWO -DTHREE=3 -DFOUR
 scons: `.' is up to date.
-</screen-->
+</screen>
 
 <para>
 See &cv-link-CPPDEFINES; for more details.

SCons/Tool/docbook/__init__.py

@@ -69,7 +69,7 @@
 # lxml etree XSLT global max traversal depth
 #
 
-lmxl_xslt_global_max_depth = 3100
+lmxl_xslt_global_max_depth = 3600
 
 if has_lxml and lmxl_xslt_global_max_depth:
     def __lxml_xslt_set_global_max_depth(max_depth):