If the supposed return address is a pointer to the beginning of an
executable page, and the page before is an oversized file mapping,
like vvar before vdso, the backtrace would be terminated by SIGBUS.
To avoid this, check not for readable, but for executable instead.
This attempts to organize all needed items and places appropriate defines
in a single location. Redundant comparisons/defines were removed when noticed.
A few caveats, however:
* This may be better targeted towards the 5_1_0 branch instead of master right now.
* Cmake will run a little slower on configuration/generation now. That's due to the sanity checks it runs.
* There are some more checks to be added later, but this should be a solid start.
Thanks to the [libical](https://github.com/libical/libical) project for inspiration.
This is meant to be a safer alternative since
NULL can often be 0. Let's not rely on that.
And yes, I know this is a lot of files. This is
a safer thing to do in big commits vs for loops.
Arusekk