helpcontent2 not building: SOLARSRC undefined

Dear LibreOffice developers,

After successfully building LO on Sunday, the most recent pull from Git 
left me with a failed build. I have re-run ./autogen.sh, but did not 
have time to 'make clean && make'. I did 'make helpcontent2.clean && 
make helpcontent2', but to no avail. The failure log is attached.

It seems that $(SOLARSRC) is undefined at 
helpcontent2/source/auxiliary/makefile.mk:102, and indeed this variable 
is missing from config_host.mk. I'm at a loss as to how to fix this.

Gert

log for /home/gert/tmp/libreoffice-core/helpcontent2/source/auxiliary
echo aux_langdirs:=en-US > ../../unxlngi6.pro/inc/aux_langs.mk
echo help_exist:=en-US > ../../unxlngi6.pro/inc/help_exist.mk
rm -f ../../unxlngi6.pro/bin/helpimg.ilst
/usr/bin/perl ../../helpers/create_ilst.pl -dir=/icon-themes/galaxy/res/helpimg > 
../../unxlngi6.pro/bin/helpimg.ilst.unxlngi6.pro
mv ../../unxlngi6.pro/bin/helpimg.ilst.unxlngi6.pro ../../unxlngi6.pro/bin/helpimg.ilst
Cannot find /icon-themes/galaxy/res/helpimg.

create_ilst.pl -dir=directory [-pre=string] 
-dir  root directory for the help images to
    be searched (default=current dir)
    Only *.png will be found.
-pre  define directory prefix (default=helpimg)
  

dmake:  Error code 2, while making '../../unxlngi6.pro/bin/helpimg.ilst'