configure: add support for pseudo-"in source tree" builds

Meson requires the build dir to be separate from the source tree. Many
people are used to just running "./configure && make" though and the
meson conversion breaks that.

This introduces some backcompat support to make it appear as if an
"in source tree" build is being done, but with the results in the
"build/" directory. This allows "./configure && make" to work as it
did historically, albeit with the output binaries staying under build/.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

.gitignore

@@ -1,3 +1,5 @@
+/GNUmakefile
+/build/
 /.doctrees
 /config-devices.*
 /config-all-devices.*

configure

@@ -11,6 +11,55 @@ unset CLICOLOR_FORCE GREP_OPTIONS
 # Don't allow CCACHE, if present, to use cached results of compile tests!
 export CCACHE_RECACHE=yes
 
+# make source path absolute
+source_path=$(cd "$(dirname -- "$0")"; pwd)
+
+if test "$PWD" = "$source_path"
+then
+    echo "Using './build' as the directory for build output"
+
+    MARKER=build/auto-created-by-configure
+
+    if test -e build
+    then
+        if test -f $MARKER
+        then
+           rm -rf build
+        else
+            echo "ERROR: ./build dir already exists and was not previously created by configure"
+            exit 1
+        fi
+    fi
+
+    mkdir build
+    touch $MARKER
+
+    cat > GNUmakefile <<'EOF'
+# This file is auto-generated by configure to support in-source tree
+# 'make' command invocation
+
+ifeq ($(MAKECMDGOALS),)
+recurse: all
+endif
+
+.NOTPARALLEL: %
+%: force
+	@echo 'changing dir to build for $(MAKE) "$(MAKECMDGOALS)"...'
+	@$(MAKE) -C build -f Makefile $(MAKECMDGOALS)
+	@if test "$(MAKECMDGOALS)" = "distclean" && \
+	    test -e build/auto-created-by-configure ; \
+	then \
+	    rm -rf build GNUmakefile ; \
+	fi
+force: ;
+.PHONY: force
+GNUmakefile: ;
+
+EOF
+    cd build
+    exec $source_path/configure "$@"
+fi
+
 # Temporary directory used for files created while
 # configure runs. Since it is in the build directory
 # we can safely blow away any previous version of it
@@ -279,9 +328,6 @@ ld_has() {
     $ld --help 2>/dev/null | grep ".$1" >/dev/null 2>&1
 }
 
-# make source path absolute
-source_path=$(cd "$(dirname -- "$0")"; pwd)
-
 if printf %s\\n "$source_path" "$PWD" | grep -q "[[:space:]:]";
 then
   error_exit "main directory cannot contain spaces nor colons"