qb: Improve the gnu99 compiler check.

[orbea]

Description

This attempts to improve the gnu99/c99 compiler check.

Reviewers

@twinaphex Please read this to make sure you agree with this logic.

Currently RetroArch checks if -std=gnu99 works or will error during configure when it doesn't. Then in the Makefile it will use -std=gnu99 unless the compiler is icc where it will use -std=c99.

With this PR RetroArch will check if -std=gnu99 works and then will check -std=c99 if the previous check has failed. If both checks fail it will then error during configure. This allows removing the compiler check in the Makefile which will presumably always have the working compiler flag.

This will also remove the GNU90_BUILD check in the Makefilewhich is entirely unused elsewhere and doesn't seem all that useful.

First does anyone actually use icc to compile RetroArch? Both gcc and clang are much better choices today.

Second should we be using -std=gnu99 instead of -std=c99? Both seem to compile and work without noticeable difference.