Fix bun_dependency_versions.h regenerating on every CMake run (#22985)

## Summary
- Fixes unnecessary regeneration of `bun_dependency_versions.h` on every
CMake run
- Only writes the header file when content actually changes

## Test plan
Tested locally by running CMake configuration multiple times:
1. First run generates the file (shows "Updated dependency versions
header")
2. Subsequent runs skip writing (shows "Dependency versions header
unchanged")
3. File modification timestamp remains unchanged when content is the
same
4. File is properly regenerated when deleted or when content changes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

cmake/tools/GenerateDependencyVersions.cmake

@@ -181,12 +181,23 @@ function(generate_dependency_versions_header)
   string(APPEND HEADER_CONTENT "}\n")
   string(APPEND HEADER_CONTENT "#endif\n\n")
   string(APPEND HEADER_CONTENT "#endif // BUN_DEPENDENCY_VERSIONS_H\n")
-  
-  # Write the header file
+
+  # Write the header file only if content has changed
   set(OUTPUT_FILE "${CMAKE_BINARY_DIR}/bun_dependency_versions.h")
-  file(WRITE "${OUTPUT_FILE}" "${HEADER_CONTENT}")
-  
-  message(STATUS "Generated dependency versions header: ${OUTPUT_FILE}")
+
+  # Read existing content if file exists
+  set(EXISTING_CONTENT "")
+  if(EXISTS "${OUTPUT_FILE}")
+    file(READ "${OUTPUT_FILE}" EXISTING_CONTENT)
+  endif()
+
+  # Only write if content is different
+  if(NOT "${EXISTING_CONTENT}" STREQUAL "${HEADER_CONTENT}")
+    file(WRITE "${OUTPUT_FILE}" "${HEADER_CONTENT}")
+    message(STATUS "Updated dependency versions header: ${OUTPUT_FILE}")
+  else()
+    message(STATUS "Dependency versions header unchanged: ${OUTPUT_FILE}")
+  endif()
   
   # Also create a more detailed version for debugging
   set(DEBUG_OUTPUT_FILE "${CMAKE_BINARY_DIR}/bun_dependency_versions_debug.txt")