Change the default value of fNoStackChecks

Stack checks are not really used currently, the variable and compiler flag were
added to support turning off qthread guard pages. While working on turning off
qthreads guard pages I noticed the default value was wrong. We want stack
checks on by default, and only want to turn them off if the user requested
no-stack-checks (or implicitly through --no-checks and --fast)
Disable qthread guard pages if user compiled with stack checks off

Turn off guard pages if the user turned off stack checks. That can happen
either explicitly through (--no-stack-checks) or implicitly through --no-checks
and thus --fast. Either way, CHPL_STACK_CHECKS (variable that gets generated to
chpl_compilation_config.h) gets set to 0. This adds an extern reference to that
variable and a check for it to determine if we should set qthread guard pages
off. Note that we do NOT override if the user explicitly set either of the
qthread guard pages env vars.
Remove extern declaration of CHPL_STACK_CHECKS from tasks-qthreads.c

This extern (actual definition in the generated code) is now in
runtime/include/chplcgfns.h. This is the proper place for it so it was added
there, and can now be removed from tasks-qthreads.c
Merge pull request #146 from ronawho/qthreads-stack-check

Bind qthreads guard pages to CHPL_STACK_CHECKS

We want to disable qthread guard pages if the user requested stack checks be
off at compile time. The infrastructure for adding a stack checks option to the
compiler and generated code was added previously and this just uses that to
actually turn QTHREAD_GUARD_PAGES off (unless the user explicitly set
QTHREAD_GUARD_PAGES themselves.)

This also changes the default value of fNoStackChecks checks since we want 
them on by default.