docs: speed up incremental builds

On each documentation build (‘make html’), doxygen regenerates XML
files. In addition to that, gen-dxd.py regenerates API reference
files under _build/inc/. This results in Sphinx flagging about half
of the input files as modified, and incremental builds taking long
time.

With this change, XML files generated by Doxygen are copied into
docs/xml_in directory only when they are changed. Breathe is pointed
to docs/xml_in directory instead of docs/xml. In addition to that,
gen-dxd.py is modified to only write to the output file when contents
change.

Overall, incremental build time (with no source files changed) is
reduced from ~7 minutes to ~8 seconds (on a particular OS X
computer).

Due to the way Breathe includes Doxygen XML files, there is still
going to be a massive rebuild every time functions, enums, macros,
structures are added or removed from the header files scanned
by Doxygen, but at least individual .rst files can be edited
at a much faster pace.

docs/link-roles.py

@@ -2,7 +2,7 @@
 
 import re
 from docutils import nodes
-from repo_util import run_cmd_get_output
+from local_util import run_cmd_get_output
 
 def get_github_rev():
     path = run_cmd_get_output('git rev-parse --short HEAD')