[OpenMP][NFC] Use CMake Variables

[atmnpatel]

Summary

This patchs adds the use of CMake variables to add subdirectories and
include directories. In addition, it renames CMake variable to a more
meaningful name that I intend to also define in its parent scope in a
later patch. Both of these variables will be used to have a clean CMake
for the upcoming remote offloading plugin.

[JonChesterfield]

LGTM. Like the explicit location of source files. Variables are namespaced with the same prefix as others in the file.

[atmnpatel]

Updated commit message, sanity ping for pre-merge buildbot.

openmp/libomptarget/CMakeLists.txt

if(LIBOMPTARGET_CMAKE_BUILD_TYPE MATCHES debug)
   option(LIBOMPTARGET_ENABLE_DEBUG "Allow debug output with the environment variable LIBOMPTARGET_DEBUG=1" ON)
 else()
   option(LIBOMPTARGET_ENABLE_DEBUG "Allow debug output with the environment variable LIBOMPTARGET_DEBUG=1" OFF)
 endif()
 if(LIBOMPTARGET_ENABLE_DEBUG)
   add_definitions(-DOMPTARGET_DEBUG)
 endif()
 
-include_directories(include)
+set(LIBOMPTARGET_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
+include_directories(${LIBOMPTARGET_INCLUDE_DIR})
 
 # Build target agnostic offloading library.
-add_subdirectory(src)
+set(LIBOMPTARGET_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src)
+add_subdirectory(${LIBOMPTARGET_SRC_DIR})
 
 # Retrieve the path to the resulting library so that it can be used for 
 # testing.
 get_target_property(LIBOMPTARGET_LIBRARY_DIR omptarget LIBRARY_OUTPUT_DIRECTORY)
 if(NOT LIBOMPTARGET_LIBRARY_DIR)
   set(LIBOMPTARGET_LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 endif()
 

openmp/libomptarget/src/CMakeLists.txt

 ##===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 ##===----------------------------------------------------------------------===##
 #
 # Build offloading library libomptarget.so.
 #
 ##===----------------------------------------------------------------------===##
 
 libomptarget_say("Building offloading runtime library libomptarget.")
 
-set(src_files
-  api.cpp
-  device.cpp
-  interface.cpp
-  MemoryManager.cpp
-  rtl.cpp
-  omptarget.cpp
+set(LIBOMPTARGET_SRC_FILES
+  ${CMAKE_CURRENT_SOURCE_DIR}/api.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/device.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/interface.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/MemoryManager.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/rtl.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/omptarget.cpp
 )
 
 # Build libomptarget library with libdl dependency. Add LLVMSupport
 # dependency if building in-tree with profiling enabled.
 if(OPENMP_STANDALONE_BUILD OR (NOT OPENMP_ENABLE_LIBOMPTARGET_PROFILING))
-  add_library(omptarget SHARED ${src_files})
+  add_library(omptarget SHARED ${LIBOMPTARGET_SRC_FILES})
   target_link_libraries(omptarget
     ${CMAKE_DL_LIBS}
     "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/exports")
 else()
   set(LLVM_LINK_COMPONENTS
     Support
     )
-  add_llvm_library(omptarget SHARED ${src_files}
+  add_llvm_library(omptarget SHARED ${LIBOMPTARGET_SRC_FILES}
       LINK_LIBS ${CMAKE_DL_LIBS}
       "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/exports"
       )
   target_compile_definitions(omptarget PUBLIC OMPTARGET_PROFILE_ENABLED)
 endif()
 
 # Install libomptarget under the lib destination folder.
 install(TARGETS omptarget LIBRARY COMPONENT omptarget
   DESTINATION "${OPENMP_INSTALL_LIBDIR}")