Merge branch 'bw/autoconf'

* bw/autoconf:
  configure: rework pthread handling to allow for user defined flags
  configure: make iconv tests aware of user arguments
  configure: asciidoc version test cleanup
  configure: wrap some library tests with GIT_STASH_FLAGS
  configure: add macros to stash FLAG variables
  configure: reorganize flow of argument checks
  configure: ensure settings from user are also usable in the script