- fix mtr.pl to look in build dir
 - change mtr.pl  to set environment variable NDB_MEMCACHE_STARTED thus avoid
   duplicated code in include/have_memcache.inc(which also didn't look in correct dir).
- slightly change mtr.pl logic to first look for ndb_engine.so and use that as the way
  to determine if NDB memcached is supported.
 - fix problem in memcached_path.pl with difference between cmake and perl
  variables. The former may be set to "1", "ON", "TRUE" etc while configuring the product
  and perl should not need to know -> translate"cmake boolean" to 1 or 0 in cmake for easier
  interpretation by perl.
 - Improve comments slightly