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/xyz/openbmc_project/Software/Image/meson.build b/xyz/openbmc_project/Software/Image/meson.build
index 293efa0..4ae382f 100644
--- a/xyz/openbmc_project/Software/Image/meson.build
+++ b/xyz/openbmc_project/Software/Image/meson.build
@@ -2,7 +2,7 @@
     'error.hpp',
     capture: true,
     command: [
-        sdbuspp,
+        sdbusplusplus_prog,
         '-r', meson.source_root(),
         'error',
         'exception-header',
@@ -18,7 +18,7 @@
     'error.cpp',
     capture: true,
     command: [
-        sdbuspp,
+        sdbusplusplus_prog,
         '-r', meson.source_root(),
         'error',
         'exception-cpp',