Add missing generated variables to runtime/include/chplcgfns.h

runtime/include/chplcgfns.h is home to extern variables and function
declarations that are defined in the generated code. It hasn't been updated in
a while so I added all of the current ones and added a note in codegen that new
values should be placed in this file.

I ran into this when I wanted to check CHPL_STACK_CHECKS. I didn't originally
realize this file existed and while adding CHPL_STACK_CHECKS I figured I'd
update all of them.
Merge pull request #147 from ronawho/update-externs-for-generated-vars

Add missing generated variables to runtime/include/chplcgfns.h

runtime/include/chplcgfns.h is home to extern variables and function
declarations that are defined in the generated code. It hasn't been updated in
a while so I added all of the current ones and added a note in codegen that new
values should be placed in this file.

I ran into this when I wanted to check CHPL_STACK_CHECKS. I didn't originally
realize this file existed and while adding CHPL_STACK_CHECKS I figured I'd
update all of them.
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.