Define VERSION in configure, and add git describe output if available.

The client release version is now defined in configure.in rather than
source/irc.c.  The gitversion.sh script is used to automatically add a
suffix to the version number for versions of the client built from git.

Also removed AC_REVISION() from configure.in, because $Revision$ doesn't
make sense with git.

gitversion.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+test $# -gt 0 || exit
+
+tag="$1"
+
+if test $# -gt 1; then
+	cd $2 || exit
+fi
+
+test -d .git || exit
+
+desc=`git describe --match "$tag" 2>/dev/null` || exit
+
+echo "$desc" | sed -e "s@^$tag@@"
+