- get platform switch logic out of shell script
- platform specific stuff should be in $(GBUILDDIR)/platform, if
  possible
- do not suggest shell-polluting exports, which might lead to confusion
  later, suggest setting the var as a one-off addition to the make
  command
- move exception caching down, as it is assumed to be used rather
  rarely

Change-Id: I01db3ca07582a6c6213ebac86fddabb28c925d50
Reviewed-on: https://gerrit.libreoffice.org/19976Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: jan iversen <jani@documentfoundation.org>
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>