[cmake] Fix export-files when building in-source

In-source builds were broken due to a mistake in the export-files
logic. Even though this commit resolves the problem, it is recommended
building out-of-source.

project/cmake/CMakeLists.txt

@@ -60,6 +60,9 @@ set(CORE_BUILD_DIR build)
 
 message(STATUS "Source directory: ${CORE_SOURCE_DIR}")
 message(STATUS "Build directory: ${CMAKE_BINARY_DIR}")
+if(CMAKE_BINARY_DIR STREQUAL CORE_SOURCE_DIR)
+  message(WARNING "In-source build detected. It is recommended to build out-of-source.")
+endif()
 
 include(modules/extra/ECMEnableSanitizers.cmake)
 include(scripts/common/GeneratorSetup.cmake)

project/cmake/scripts/common/Macros.cmake

@@ -77,17 +77,21 @@ function(copy_file_to_buildtree file relative)
   string(REPLACE "${relative}/" "" outfile ${file})
   get_filename_component(outdir ${outfile} DIRECTORY)
 
-  if(NOT TARGET export-files)
-    file(REMOVE ${CMAKE_BINARY_DIR}/${CORE_BUILD_DIR}/ExportFiles.cmake)
-    add_custom_target(export-files ALL COMMENT "Copying files into build tree"
-                      COMMAND ${CMAKE_COMMAND} -P ${CMAKE_BINARY_DIR}/${CORE_BUILD_DIR}/ExportFiles.cmake)
-  endif()
-  if(NOT ${CORE_SOURCE_DIR} MATCHES ${CMAKE_BINARY_DIR})
+  if(NOT CMAKE_BINARY_DIR STREQUAL CORE_SOURCE_DIR)
+    if(NOT TARGET export-files)
+      file(REMOVE ${CMAKE_BINARY_DIR}/${CORE_BUILD_DIR}/ExportFiles.cmake)
+      add_custom_target(export-files ALL COMMENT "Copying files into build tree"
+                        COMMAND ${CMAKE_COMMAND} -P ${CMAKE_BINARY_DIR}/${CORE_BUILD_DIR}/ExportFiles.cmake)
+    endif()
     if(VERBOSE)
       message(STATUS "copy_file_to_buildtree - copying file: ${file} -> ${CMAKE_BINARY_DIR}/${outfile}")
     endif()
     file(APPEND ${CMAKE_BINARY_DIR}/${CORE_BUILD_DIR}/ExportFiles.cmake
          "file(COPY \"${file}\" DESTINATION \"${CMAKE_BINARY_DIR}/${outdir}\")\n")
+  else()
+    if(NOT TARGET export-files)
+      add_custom_target(export-files ALL)
+    endif()
   endif()
   if(NOT arg_NO_INSTALL)
     list(APPEND install_data ${outfile})