Set distro name using --with-pkgconfig configure option
    
    Instead of placing the distro name (e.g. 'Fedora Linux') into the
    version.in file, use the --with-pkgconfig configure option to set the
    disto name.
    
    This does change GDB's version output slightly.   Prior to this commit
    we might expect to see something like this:
    
      $ gdb --version
      GNU gdb (GDB) Fedora Linux 13.1-1-fc38
      ...
    
    Now we'll see this:
    
      $ gdb --version
      GNU gdb (Fedora Linux) 13.1-1-fc38
      ...
    
    Notice that 'Fedora Linux' moved inside the parenthesis.
    
    There are other benefits to this change, the Python gdb.VERSION
    string will now contain '13.1-1-fc38' instead of 'Fedora Linux
    13.1-1-fc38', this was reported as an issue in this bug:
    
      https://bugzilla.redhat.com/show_bug.cgi?id=2179554
    
    I've also removed the change to gdb.gdb/selftest.exp as I don't
    believe that this was ever being used -- when this test was run we
    should have hit an earlier case.
    
    I have added a brand new test that checks our version string.  I've
    placed this new test into a separate file, so we should (hopefully)
    not have any issues maintaining this as an out of tree test.
    
    For implementation, by default we make use of the %dist_name macro,
    however, this is not defined on RHEL 9 or earlier, so to support
    reusing this commit on RHEL systems, I also detect the %rhel macro,
    and in that case use a hard-coded string.