Implement fallback if Glib::Regex class is missing (#695279)

GParted fails to compile on RHEL/CentOS 5.9 because it doesn't provide
the Glib::Regex class.  Glib::Regex class requires glibmm >= 2.14,
however RHEL/CentOS 5.9 only provides glibmm 2.12.

Add an autoconf check for the Glib::Regex class and fallback code using
the POSIX regex function.  Fall back code is the same as that used prior
to commit:
    b6f1c56f
    Enhance regexp_label method to handle unicode characters

Bug #695279 - GParted doesn't compile on RHEL / CentOS 5.9