[Offload] Fix CMake detection when it is not found (#90729)

Summary:
This variable could be unset if not found or when building standalone.
We should check for that and set it to true or false.

Fixes: #90708

Author: jhuber6

offload/CMakeLists.txt

@@ -302,7 +302,12 @@ endif()
 
 pythonize_bool(LIBOMPTARGET_OMPT_SUPPORT)
 
-set(LIBOMPTARGET_GPU_LIBC_SUPPORT ${LLVM_LIBC_GPU_BUILD} CACHE BOOL
+if(${LLVM_LIBC_GPU_BUILD})
+  set(LIBOMPTARGET_HAS_LIBC TRUE)
+else()
+  set(LIBOMPTARGET_HAS_LIBC FALSE)
+endif()
+set(LIBOMPTARGET_GPU_LIBC_SUPPORT ${LIBOMPTARGET_HAS_LIBC} CACHE BOOL
     "Libomptarget support for the GPU libc")
 pythonize_bool(LIBOMPTARGET_GPU_LIBC_SUPPORT)