Merge branch '3.4-maint'

* 3.4-maint:
  Use double when calculating cache thresholds
  Add a 32-bit build target, using multilib (-m32)
  Convert float config to double, add rounding
  Upgrade clang to 5.0, for the Travis docker
  Make sure to export ASAN_OPTIONS properly
  Add small helper to run all Travis tests
  Make sure to call configure with right path
  Add a travis-build container, for running locally
  Look for -fdebug-prefix-map feature explicitly
  Improve the Travis build matrix
  Fix log printing of before/after cleanup
  Remove irrelevant variable assignment in wipe_dir
  win32: Fix CCACHE_COMPILERCHECK=mtime test case