use -MMD if the compiler supports it

it's better than the previous Makefile.depend approach since this
automatically adapts to the included headers without requiring
manual intervention to regen the list.

Makefile

@@ -70,7 +70,7 @@ static: ${GMID_OBJS} ${GE_OBJS} ${GG_OBJS}
 	${CC} ${GG_OBJS} -o gg ${LDFLAGS} ${STATIC}
 
 clean:
-	rm -f *.o compat/*.o y.tab.c y.tab.h y.output gmid ge gg
+	rm -f *.[do] compat/*.[do] y.tab.c y.tab.h y.output gmid ge gg
 	rm -f compile_flags.txt
 	${MAKE} -C regress clean
 
@@ -130,3 +130,23 @@ ${DISTNAME}.tar.gz: ${DISTFILES}
 	${MAKE} -C regress	DESTDIR=${PWD}/.dist/${DISTNAME}/regress dist
 	cd .dist/ && tar zcf ../$@ ${DISTNAME}
 	rm -rf .dist/
+
+# -- dependencies --
+
+-include config.d
+-include dirs.d
+-include fcgi.d
+-include gg.d
+-include gmid.d
+-include iri.d
+-include log.d
+-include logger.d
+-include mime.d
+-include proc.d
+-include proxy.d
+-include puny.d
+-include sandbox.d
+-include server.d
+-include utf8.d
+-include utils.d
+-include y.tab.d

configure

@@ -240,6 +240,10 @@ fi
 # --------
 # tests for config.h
 
+if runtest noop MMD -MMD; then
+	CFLAGS="${CFLAGS} -MMD"
+fi
+
 runtest err		ERR		|| true
 runtest explicit_bzero	EXPLICIT_BZERO	|| true
 runtest freezero	FREEZERO	|| true