Makefile: Forbid out-of-tree build from a source tree that has been built in

If we try to do an out-of-tree build but the source tree we're building from
has been used in the past for an in-tree build then things will go
confusingly wrong. Specifically, some parts of the build process will pull
in generated files from the old in-tree build (because SRC_PATH is on
the vpath). Diagnose this situation so we can produce a useful error
message and tell the user how to fix it (run distclean in the source tree).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

Makefile

@@ -8,6 +8,17 @@ ifneq ($(wildcard config-host.mak),)
 # Put the all: rule here so that config-host.mak can contain dependencies.
 all:
 include config-host.mak
+
+# Check that we're not trying to do an out-of-tree build from
+# a tree that's been used for an in-tree build.
+ifneq ($(realpath $(SRC_PATH)),$(realpath .))
+ifneq ($(wildcard $(SRC_PATH)/config-host.mak),)
+$(error This is an out of tree build but your source tree ($(SRC_PATH)) \
+seems to have been used for an in-tree build. You can fix this by running \
+"make distclean && rm -rf *-linux-user *-softmmu" in your source tree)
+endif
+endif
+
 include $(SRC_PATH)/rules.mak
 config-host.mak: $(SRC_PATH)/configure
 	@echo $@ is out-of-date, running configure