mammoth patch:  update build system, scripts and code for universal build

Ouch, another biggie.  Most of this is easy to debug when it breaks,
luckily.

- update build system for universal build
  - '--with-threads=<foo>' removed; now use 'make threads=<foo>'
  - configure.in/Makefile.inc breaks out all(?) flavor-specific variables

- update scripts for universal build
  - scripts/rtapi.conf
    - per-flavor variables broken out, e.g. MODULE_DIR, KERNEL_VERS
    - settings removed from scripts/realtime to rtapi.conf
    - outdated MODPATH_foo stuff removed
  - scripts/realtime
    - most settings come from rtapi.conf
    - kernel checking updated
  - FIXME when RTAPI modules live in separate MODULE_DIRs, tweaks needed
  - FIXME half-assed job updating linuxcncconfig.py

- update code for universal build
  - MODULE_EXT is flavor dependent
    - replaced with rtapi_compat functions in most cases

- remove outdated cruft
  - math detection: assuming that this is no longer an issue
    - loads of code in configure.in; mathstubs.c
  - module path detection:  long obsolete, and changing again
  - variable proliferation:  xeno_config
  - RTDIR variable/macro almost gone
    - now used only by latencyplot (which only works with RTAI) FIXME
  - MODEXT/MODULE_EXT now comes from rtapi_compat code
  - RTPREFIX, RTNAME, RTAI variables
  - old thread detection code in configure.in; no more AC_SUBST(THREADS)
  - --run-in-place and other warnings