[OpenMP][Offload] - Ensure OPENMP_STANDALONE_BUILD is defined (#94801)

Without a value set conditional checks like
if(NOT ${OPENMP_STANDALONE_BUILD})
will not be able to evaluate to true.
Fixes issue introduced from PR #93463, which did not allow the OMPT
variable to be propogated up to offload during a runtimes build.

offload/CMakeLists.txt

@@ -7,6 +7,8 @@ set(LLVM_SUBPROJECT_TITLE "liboffload")
 if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
   set(OPENMP_STANDALONE_BUILD TRUE)
   project(offload C CXX ASM)
+else()
+  set(OPENMP_STANDALONE_BUILD FALSE)
 endif()
 
 # Check that the library can actually be built.

openmp/CMakeLists.txt

@@ -13,6 +13,8 @@ list(INSERT CMAKE_MODULE_PATH 0
 if (OPENMP_STANDALONE_BUILD OR "${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
   set(OPENMP_STANDALONE_BUILD TRUE)
   project(openmp C CXX ASM)
+else()
+  set(OPENMP_STANDALONE_BUILD FALSE)
 endif()
 
 # Must go below project(..)