Build with -std=gnu99 instead of -std=c99

[fweimer-rh]

The latter activates strict POSIX mode with glibc, which disables the declaration of usleep in <unistd.h>, leading to an implicit function declaration.

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

[posva]

Thanks a lot for the links, I will try to review this and others during the holidays!

[Kreijstal]

that would create libc dependency :((((