build: support building with subprojects

Enable everything to build outside of an OE-SDK by leveraging
subproject wrap files.  Needed to update C++ to C++20 as well
in order to allow phosphor-logging to compile.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I43f5fcd53ef3ac024d7b5ce50fd035903d08bab0
diff --git a/subprojects/phosphor-dbus-interfaces.wrap b/subprojects/phosphor-dbus-interfaces.wrap
new file mode 100644
index 0000000..935a8b2
--- /dev/null
+++ b/subprojects/phosphor-dbus-interfaces.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/openbmc/phosphor-dbus-interfaces.git
+revision = HEAD