[offload] Define LIBOMP_*_DIR in standalone builds

Define `LIBOMP_INCLUDE_DIR` and `LIBOMP_LIBRARY_DIR` for testing
in standalone builds.

Author: mgorny

offload/CMakeLists.txt

@@ -288,6 +288,17 @@ if(OPENMP_STANDALONE_BUILD)
     REQUIRED
   )
 
+  find_path (
+    LIBOMP_INCLUDE_DIR
+    NAMES
+      omp.h
+    HINTS
+    ${COMPILER_RESOURCE_DIR}/include
+    ${CMAKE_INSTALL_PREFIX}/include
+  )
+
+  get_filename_component(LIBOMP_LIBRARY_DIR ${LIBOMP_STANDALONE} DIRECTORY)
+
   set(OPENMP_TEST_FLAGS "" CACHE STRING
     "Extra compiler flags to send to the test compiler.")
   set(OPENMP_TEST_OPENMP_FLAGS ${OPENMP_TEST_COMPILER_OPENMP_FLAGS} CACHE STRING