Previously: unconditionally use the size from ECL_OPT_C_STACK_SIZE if
it is larger than the OS provided value and if possible resize the
stack at runtime.
Now: Use ECL_OPT_C_STACK_SIZE if provided. Otherwise, use a) the OS
provided value if we can resize the stack at runtime (for Unix) or b)
the new build option ECL_DEFAULT_C_STACK_SIZE (1 MB) if we can set the
stack size at link time (for Windows).