Call libuev configure script if local libuev is used

Same as for libite, libuev has also changed to autotools, so
call its autogen.sh and configure script in cascade from our
own.  Aslo add safeguards to clean/distclean targets so we
do not attempt to enter the libuev subdirectory if it's not
configured.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile

@@ -147,7 +147,9 @@ clean:
 ifndef LIBITE
 	+$(MAKE) -C libite  $@
 endif
+ifndef LIBUEV
 	+$(MAKE) -C libuev  $@
+endif
 	-@$(RM) $(OBJS) $(DEPS) $(EXEC) 2>/dev/null
 
 distclean: clean
@@ -155,7 +157,9 @@ distclean: clean
 ifndef LIBITE
 	+$(MAKE) -C libite  $@
 endif
+ifndef LIBUEV
 	+$(MAKE) -C libuev  $@
+endif
 	-@$(RM) $(JUNK) config.mk config.h unittest *.o *.html 2>/dev/null
 
 check: