Ensure that functions are declared before use for compatibility
with newer compiler versions.
There are three fixes here:
* Use return instead of exit in configure.
* Include error.h for the error() function.
* This causes a format security error, which we avoid by
indirecting through %s.
Related to:
<http://fedoraproject.org/wiki/Changes/PortingToModernC>
<http://fedoraproject.org/wiki/Toolchain/PortingToModernC>