Integrate doxygen documentation building into autoconf/automake process
diff --git a/configure.ac b/configure.ac
index b95c8cc..a77e3e8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,6 +20,9 @@
 AC_HEADER_STDC
 AC_CHECK_HEADERS(execinfo.h sys/select.h sys/socket.h syslog.h ctype.h)
 
+AC_PATH_PROG(DOXYGEN,doxygen,false)
+AM_CONDITIONAL(HAVE_DOXYGEN, test $DOXYGEN != false)
+
 # The following test is taken from WebKit's webkit.m4
 saved_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -fvisibility=hidden "