Check if the macro AC_PROG_CC_C99 actually made the compiler accept C99

[Danack]

Same idea as #6314 - check that the compiler support C99 so users get a decent error message, rather than a cryptic one.

This also need to be checked on a system that has a non-C99 compiler, and I don't have one to hand.

[Danack]

For reference, the original reason for this PR was https://bugs.php.net/bug.php?id=80211

[Girgias]

Does this still make sense?

[Danack]

Meh.

The value for it is dropping over time. Not many people are complaining about PHP not being <C99. On the other hand it's cheap, and it gives a clear error message rather than leaving people wondering what is happening.

Assuming it doesn't break anything* it's probably worth committing. Comparing user time over the next ten years:

• Time saved from people wondering wtf vs time taken up.

• 100 people * 10 minutes * 60 seconds in a minute vs 0.00001 seconds per build * 10000 people who build PHP from source * 200 times a year * 10 years.

• 60,000 seconds vs 200 seconds.

So....that seems worth it?

*ha.