- do concat for json in C++, everything else seems fragile on Windows
- have APPEND vars separately
- check that gbuildtoide work on modules without a full build (modulo
  some blacklisted "creative" ones)

Change-Id: I6fe267fee7d1b77d758072303729387dfeb8e6c8
Reviewed-on: https://gerrit.libreoffice.org/30293Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>