Enable use of .depend on gmake. Also, generate extra dependencies for our shared libraries.

mk/depend.mk

@@ -1,8 +1,5 @@
-# This only works for make implementations that always include a .depend if
-# it exists. Only GNU make does not do this.
-
+# Generate .depend
 # Copyright 2008 Roy Marples <roy@marples.name>
-# All rights reserved. Released under the 2-clause BSD license.
 
 CLEANFILES+=	.depend
 IGNOREFILES+=	.depend
@@ -10,4 +7,9 @@ IGNOREFILES+=	.depend
 .depend: ${SRCS}
 	${CC} ${CPPFLAGS} -MM ${SRCS} > .depend
 
-depend: .depend
+depend: .depend extra_depend
+
+# Nasty hack for gmake which does not automatically include .depend
+# if it exists, unlike every other make implementation.
+INC_DEPEND= $(shell if test -e .depend; then echo ".depend"; else echo ""; fi)
+include ${INC_DEPEND}