Makefile.modinc:  frame size checks only needed for kbuild

https://github.com/zultron/linuxcnc/issues/13
Makefile.modinc:  update kbuild 'modules' recipe to match UB
rtapi/rtapi_export.h needs to be installed
Makefile.modinc.in:  Remove RTFLAGS, defined in Makefile.inc
Makefile.modinc:  Update BUILD_SYS variable name
Fix out-of-tree comps building, part 1 (still broken)

Makefile.modinc needs to know what flavor to build for when building
out of tree.  This commit gets that infrastructure in place.

- Pick a realistic 'default' for $(threads) in Makefile.inc
  - if the 'flavor' executable is available in $PATH, use that default
  - otherwise, just pick the first in the list

- Compute $(RTDIR_EXT) in Makefile.inc, e.g. rtai-kernel/2.6.38-8.rtai

- Install module in RTLIB_DIR in Makefile.modinc where it can be found

- Makefile needs KERNEL_VERS computed up front

FIXME:  still broken:

- Makefile.modinc doesn't have kernel version for kthreads
  - this should default to the running kernel

- symbols.0 test still broken
  - the 'undefined symbol: testuse' string is going to syslog,
    not stderr or dmesg where the test looks

runtests symbols.0 fix WIP

squash
Fix out-of-tree comps building, part 2 (still broken): kthreads

- Use `uname -r` for KERNEL_VERS
  - runtests for kthreads should build against running kernel
- Hack location for KERNELDIR
  - this *should* be the standard location
  - better would be to go through <FLAVOR>_THREADS_KERNEL_DIRS and
    run kernel-vers.sh against each until one matches `uname -r`.

FIXME:  still broken:

- symbols.0 test still broken
  - the 'undefined symbol: testuse' string is going to syslog,
    not stderr or dmesg where the test looks

- lots of undefined symbols
  - something still must be missing from Makefile.modinc