firefox 45 + webrtc: configure stage fails

[Timo Buhrmester <fstd.lkml%gmail.com@localhost>]

Attempting to build firefox (version 45, from the 2016Q1 branch) with
the "webrtc" option enabled gives on -current (7.99.26, i386) gives:
(Full output: http://paste.pr0.tips/GD)

| [...]
| js/src> creating ./config.status
| 
| Reticulating splines...
| Traceback (most recent call last):
|   File "./config.status", line 1067, in <module>
|     config_status(**args)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/python/mozbuild/mozbuild/config_status.py", line 175, in config_status
|     definitions = list(definitions)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/python/mozbuild/mozbuild/frontend/emitter.py", line 165, in emit
|     for out in output:
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/python/mozbuild/mozbuild/frontend/reader.py", line 1062, in read_mozbuild
|     raise bre
| mozbuild.frontend.reader.BuildReaderError: 
| ==============================
| ERROR PROCESSING MOZBUILD FILE
| ==============================
| 
| The error occurred while processing the following file:
| 
|     /pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/moz.build
| 
| The error appears to be part of the mozbuild.frontend.reader Python module itself! It is possible you have stumbled across a legitimate bug.
| 
| Traceback (most recent call last):
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/python/mozbuild/mozbuild/frontend/reader.py", line 1058, in read_mozbuild
|     metadata=metadata):
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/python/mozbuild/mozbuild/frontend/reader.py", line 1167, in _read_mozbuild
|     non_unified_sources = non_unified_sources):
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/python/mozbuild/mozbuild/frontend/gyp_reader.py", line 110, in read_from_gyp
|     params=params)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/__init__.py", line 132, in Load
|     params['parallel'])
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/input.py", line 2560, in Load
|     variables, includes, depth, check, True)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/input.py", line 441, in LoadTargetBuildFile
|     includes, depth, check, load_dependencies)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/input.py", line 365, in LoadTargetBuildFile
|     includes, True, check)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/input.py", line 238, in LoadOneBuildFile
|     aux_data, variables, includes, check)
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/input.py", line 276, in LoadBuildFileIncludesIntoDict
|     False, check),
|   File "/pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/tools/gyp/pylib/gyp/input.py", line 223, in LoadOneBuildFile
|     None)
|   File "<string>", line 200
|     ],
|      ^
| SyntaxError: invalid syntax while reading includes of /pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/webrtc/modules/modules.gyp while loading dependencies of /pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/peerconnection.gyp while trying to load /pkg_comp/obj/pkgsrc/www/firefox/default/firefox-45.0.1/media/webrtc/trunk/peerconnection.gyp
| 
| *** Error code 1
| 
| Stop.
| make[1]: stopped in /usr/pkgsrc/www/firefox
| *** Error code 1
| 
| Stop.
| make: stopped in /usr/pkgsrc/www/firefox