rxp compilation mode support

[Reto Stamm]

This bug report will be sent to the Free Software Foundation,
 not to your local site managers!!
Please write in English, because the Emacs maintainers do not have
translators to read other languages for them.

In GNU Emacs 20.7.1 (sparc-sun-solaris2.6, X toolkit)
 of Fri Nov 17 2000 on spt7
configured using `configure  --prefix /proj/swop4/gnu'

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

RXP is an xml parser and validater who's messages are not understood
by emacs -- unless the following lines are added at the end of
compilation-error-regexp-alist in compile.el


    ;; RXP - gpl xml validator from http://www.cogsci.ed.ac.uk/~richard/rxp.html
    ;; Error: Mismatched end tag: expected </geroup>, got </group>
    ;; in unnamed entity at line 71 char 8 of file:///home/reto/test/group.xml
    ("Error:.*\n.* line \\([0-9]+\\) char \\([0-9]+\\) of file://\\(.+\\)"
     3 1 2)
    

    ;; RXP
    ;; Warning: Start tag for undeclared element geroup
    ;; in unnamed entity at line 4 char 8 of file:///home/reto/test/group.xml
    ("Warning:.*\n.* line \\([0-9]+\\) char \\([0-9]+\\) of file://\\(.+\\)"
     3 1 2)