add submodule check for XMP-Toolkit-SDK at configure time

Fresh clones without --recurse-submodules would fail with cryptic
source-file-not-found errors.  Add an explicit check for the SDK's
presence in both CMake and Meson, with a clear error message telling
the user to run git submodule update --init --recursive.

Author: serghov

xmpsdk/CMakeLists.txt

@@ -1,3 +1,9 @@
+if (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/XMP-Toolkit-SDK/public/include/XMP_Const.h")
+    message(FATAL_ERROR
+        "XMP-Toolkit-SDK submodule not found. Please run:\n"
+        "  git submodule update --init --recursive")
+endif()
+
 add_library(
         exiv2-xmp OBJECT
         XMP-Toolkit-SDK/public/include/XMP_Const.h

xmpsdk/meson.build

@@ -6,6 +6,12 @@ endif
 
 sdk = 'XMP-Toolkit-SDK'
 
+fs = import('fs')
+if not fs.is_file(sdk / 'public' / 'include' / 'XMP_Const.h')
+    error('XMP-Toolkit-SDK submodule not found. Please run:\n' +
+          '  git submodule update --init --recursive')
+endif
+
 # The SDK includes expat as "third-party/expat/lib/expat.h".
 # A wrapper header at xmpsdk/third-party/expat/lib/expat.h redirects
 # this to the system expat via #include_next.