chore(cmake): honor -DMIGRATE_MODULE to add only necessary subdirectories for per-module migration builds\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

CMakeLists.txt

@@ -326,11 +326,35 @@ if(NOT RTI_FOUND)
   endforeach()
 endif()
 
-ADD_SUBDIRECTORY(TetraIF)
+# Respect MIGRATE_MODULE: when set to a relative path (e.g. "Lts/Common"), only add
-ADD_SUBDIRECTORY(Z/src)
+# the top-level subdirectories required to configure/build that module. This allows
-ADD_SUBDIRECTORY(Lts)
+# per-module migration builds via -DMIGRATE_MODULE=<path>.
+if(NOT DEFINED MIGRATE_MODULE)
+  set(MIGRATE_MODULE "" CACHE STRING "If set, only add subdirectories covering this module (path relative to repo root)")
+endif()
+
+function(NGL_ADD_SUBDIR_GUARDED subdir)
+  if(MIGRATE_MODULE STREQUAL "")
+    add_subdirectory(${subdir})
+  else()
+    # normalize to forward slashes
+    string(REPLACE "\\" "/" _mig "${MIGRATE_MODULE}")
+    string(REPLACE "\\" "/" _sub "${subdir}")
+    if(_mig MATCHES "^${_sub}(/|$)" OR _sub MATCHES "^${_mig}(/|$)")
+      message(STATUS "Including subdir ${subdir} because MIGRATE_MODULE=${MIGRATE_MODULE}")
+      add_subdirectory(${subdir})
+    else()
+      message(STATUS "Skipping subdir ${subdir} because MIGRATE_MODULE=${MIGRATE_MODULE}")
+    endif()
+  endif()
+endfunction()
+
+# Add only guarded subdirectories
+NGL_ADD_SUBDIR_GUARDED(TetraIF)
+NGL_ADD_SUBDIR_GUARDED(Z/src)
+NGL_ADD_SUBDIR_GUARDED(Lts)
 # at last, to make sure we get all targets in NGL_TS_TARGETS property
-ADD_SUBDIRECTORY(translations)
+NGL_ADD_SUBDIR_GUARDED(translations)
 #-----------------------------------------------------------------------------------------------------