[doc]: Conditionally add ocaml_doc to ALL target

Add the ocaml_doc target to the default ALL build target, but only when LLVM_BUILD_DOCS is enabled.

This addresses two issues:

1. It prevents the 'install' target from failing due to a missing ocamldoc output directory by ensuring docs are generated beforehand.

2. It avoids unnecessary build steps when documentation is not required (LLVM_BUILD_DOCS is OFF), saving build time.

Author: Acture

llvm/docs/CMakeLists.txt

@@ -136,7 +136,12 @@ if( NOT uses_ocaml LESS 0 AND LLVM_ENABLE_OCAMLDOC )
     list(APPEND odoc_files -load ${odoc_file})
   endforeach()
 
-  add_custom_target(ocaml_doc
+  set(OCAML_DOC_ADD_TO_ALL "")
+  if(LLVM_BUILD_DOCS)
+    set(OCAML_DOC_ADD_TO_ALL ALL)
+  endif()
+
+  add_custom_target(ocaml_doc ${OCAML_DOC_ADD_TO_ALL}
     COMMAND ${CMAKE_COMMAND} -E remove_directory ${CMAKE_CURRENT_BINARY_DIR}/ocamldoc/html
     COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/ocamldoc/html
     COMMAND ${OCAMLFIND} ocamldoc -d ${CMAKE_CURRENT_BINARY_DIR}/ocamldoc/html