xymon_alerting lense does not work in uClibc-based systems

Augeas initialization fails on uClibc-based systems because the xymon_alerting lense uses a non-greedy match in two regular expressions. The problem could be fixed in the following ways:

1. Require all lenses to use POSIX-compliant regular expressions only. This would be consistent with the user documentation on lense syntax.

2. Make the configure script use the gnulib regex implementation unless the host's libc supports all features used by the lenses. The exact regex syntax should be described in the documentation if non-POSIX extensions are allowed.