[offload] Permit redefining OPENMP_STANDALONE_BUILD (llvm#157253)

Permit redefining `OPENMP_STANDALONE_BUILD` to make it possible to build
offload correctly via runtimes build (i.e. build where the top-level
project is `runtimes`). This follows the same logic in `openmp`
component.

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

offload/CMakeLists.txt

@@ -4,7 +4,8 @@
 cmake_minimum_required(VERSION 3.20.0)
 set(LLVM_SUBPROJECT_TITLE "liboffload")
 
-if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
+# Permit redefining OPENMP_STANDALONE_BUILD when doing a runtimes build.
+if (OPENMP_STANDALONE_BUILD OR "${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
   set(OPENMP_STANDALONE_BUILD TRUE)
   project(offload C CXX ASM)
 else()