[NFC][libomptarget] Fixed -DLLVM_ENABLE_RUNTIMES="openmp" build.

[vzakhari]

Summary

This should fix build issues reported in https://reviews.llvm.org/D103545#2826676

[ronlieb]

worked for me

openmp/libomptarget/cmake/Modules/LibomptargetGetDependencies.cmake

if (OPENMP_STANDALONE_BUILD)
   # in an out-of-tree mode.
   find_package(LLVM REQUIRED)
   message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
   message(STATUS "Using LLVM in: ${LLVM_DIR}")
   list(APPEND LIBOMPTARGET_LLVM_INCLUDE_DIRS ${LLVM_INCLUDE_DIRS})
   list(APPEND CMAKE_MODULE_PATH ${LLVM_CMAKE_DIR})
   include(AddLLVM)
 else()
+  # Note that OPENMP_STANDALONE_BUILD is FALSE, when
+  # openmp is built with -DLLVM_ENABLE_RUNTIMES="openmp" vs
+  # -DLLVM_ENABLE_PROJECTS="openmp", but openmp build
+  # is actually done as a standalone project build with many
+  # LLVM CMake variables propagated to it.
   list(APPEND LIBOMPTARGET_LLVM_INCLUDE_DIRS
     ${LLVM_MAIN_INCLUDE_DIR} ${LLVM_BINARY_DIR}/include
     )
+  message(STATUS
+    "Using LLVM include directories: ${LIBOMPTARGET_LLVM_INCLUDE_DIRS}")
 endif()
 
 ################################################################################
 # Looking for libelf...
 ################################################################################
 
 find_path (
   LIBOMPTARGET_DEP_LIBELF_INCLUDE_DIR

openmp/libomptarget/plugins/common/elf_common/CMakeLists.txt

 #
 ##===----------------------------------------------------------------------===##
 
 add_library(elf_common OBJECT elf_common.cpp)
 
 llvm_update_compile_flags(elf_common)
 set(LINK_LLVM_LIBS LLVMBinaryFormat LLVMObject LLVMSupport)
 target_link_libraries(elf_common INTERFACE ${LINK_LLVM_LIBS} ${LIBOMPTARGET_DEP_LIBELF_LIBRARIES})
+include_directories(${LIBOMPTARGET_LLVM_INCLUDE_DIRS})
 add_dependencies(elf_common ${LINK_LLVM_LIBS})
 
 # The code uses Debug.h, which requires threads support.
 target_link_libraries(elf_common INTERFACE ${OPENMP_PTHREAD_LIB})
 
 # Expose elf_common.h directory to the users of this library.
 target_include_directories(elf_common INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})