configure.ac: Avoid implicit declaration of the exit function

[fweimer-rh]

Future C compilers are likely to reject implicit function declarations by default. This language feature was officially removed in 1999.

Related to:

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

[oetiker]

thanks