Don't use env vars for configuration time options

- As a general rule of thumb one shouldn't use environment variables
  for setting configuration options for CMake. The reason for this is
  we don't know when CMake will be executed and re-evaluate that variable.
- e.g. If we run cmake a first time with a var set, and then run make on
  a second session (with no var) and cmake is called because a
  dependency changed, the option would be disabled
- This commit removes the use of environment vars from
  src/nvim/CMakeLists.txt entirely
- Removed SKIP_UNITTEST since it could only be used to remove a target
  at configuration time (and the target was optional anyway)
- Turned SANITIZE into an option, clang-asan.sh now passes cmake
  -DSANITIZE=ON
- Removed SKIP_EXEC since it was disabling a target at configuration time
  (not being used)
Only use SANITIZE for nvim target

- Clang's Address Sanitizer options may cause problems when running
  the unit tests, restrict the use of these options to the nvim target.
CMake: Remove msgpack shared library in subdirectories.
strings.c: replace copy_spaces, copy_chars with equivalent memset.
Merge pull request #1604 from equalsraf/tb-env-configs

Don't use env vars for configuration time options.
Merge pull request #1635 from danthedeckie/master

replace copy_spaces and copy_chars functions with equivalent memset.
strcnt: remove unused parameter
Merge pull request #1642 from fwalch/msgpack-glob

CMake: Remove shared libraries for messagepack.
Remove VMS remnants
Remove dec-mcs remnants

It was (apparently) the default character encoding file used for line
printing on VMS, which is unsupported.
Remove Amiga remnants

Notes regarding the removal of specific items:

  - Aztec C: only on the Amiga.
  - mch_check_win(): doesn't exist anymore.
  - Comment in ex_cmds.c: It seems the context for this comment was
    removed, but the comment was inadvertantly left alone.