feat: Fail early if Ninja sources are missing

Add an explicit check in the top-level `CMakeLists.txt` to verify that the
`ninja-upstream` source directory is present. This helps catch configuration
issues early, especially in environments where the source was cloned without
initializing submodules.

If the directory is missing and the project is under Git, the error suggests
initializing submodules. Otherwise, it indicates an invalid source distribution.

Author: jcfr

CMakeLists.txt

@@ -4,6 +4,16 @@ project(NinjaPythonDistributions)
 
 set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_MODULE_PATH})
 
+# Verify that the Ninja source directory is available
+if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/ninja-upstream/CMakeLists.txt")
+  if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
+    set(_details "Make sure to initialize submodules: git submodule update --init --recursive.")
+  else()
+    set(_details "Source distribution appears to be incomplete or invalid.")
+  endif()
+  message(FATAL_ERROR "Missing ninja-upstream sources at [${CMAKE_CURRENT_SOURCE_DIR}/ninja-upstream]. ${_details}")
+endif()
+
 # Options
 option(RUN_NINJA_TEST "Run Ninja test suite" OFF)