[SYCL][NFC] Add an option to disable updates of external projects (#1891)

`sycl` subproject uses several "ExternalProject"s [1] within it to get
some underlying components required for build and execution, like OpenCL
Headers, OpenCL ICD and L0 Loader. However, being forced to perform
updates of them each time when cmake is re-run might be annoying in some
networks.

Added possibility to disable that auto-update:
`SYCL_EXTERNAL_PROJECTS_SKIP_AUTO_UPDATE` option was introduced, which
is inteded to control all external projects used in `sycl` subprojects.
Also, added a few more variables to provide control over each particular
project.

[1]: https://cmake.org/cmake/help/v3.13/module/ExternalProject.html

Author: AlexeySachkov

sycl/CMakeLists.txt

@@ -79,13 +79,26 @@ endif()
 
 include(ExternalProject)
 
+# By default, whenever cmake is re-run, the external project's sources will be
+# updated, which might be annoying - let's have a possibility to disable that as
+# not every external project which is used here is being updated often
+# Single toggle to control all external projects
+option(SYCL_EXTERNAL_PROJECTS_SKIP_AUTO_UPDATE
+  "Perform automatic update step for external projects (OpenCL Headers, OpenCL ICD, etc.)" OFF)
+# More precise controls for each particular external project are documented
+# near each ExternalProject_Add
+
 if( NOT OpenCL_INCLUDE_DIRS )
   message(STATUS "OpenCL_INCLUDE_DIRS is missing. Will try to download OpenCL headers from github.com")
   set(OpenCL_INCLUDE_DIRS "${CMAKE_CURRENT_BINARY_DIR}/OpenCL/inc")
+  if (NOT DEFINED SYCL_EP_OCL_HEADERS_SKIP_AUTO_UPDATE)
+    set(SYCL_EP_OCL_HEADERS_SKIP_AUTO_UPDATE ${SYCL_EXTERNAL_PROJECTS_SKIP_AUTO_UPDATE})
+  endif()
   ExternalProject_Add(ocl-headers
     GIT_REPOSITORY    https://github.com/KhronosGroup/OpenCL-Headers.git
     GIT_TAG           origin/master
-    SOURCE_DIR        "${CMAKE_CURRENT_BINARY_DIR}/OpenCL/inc"
+    UPDATE_DISCONNECTED ${SYCL_EP_OCL_HEADERS_SKIP_AUTO_UPDATE}
+    SOURCE_DIR        ${OpenCL_INCLUDE_DIRS}
     CONFIGURE_COMMAND ""
     BUILD_COMMAND     ${CMAKE_COMMAND} -E copy_directory ${OpenCL_INCLUDE_DIRS}/CL ${SYCL_INCLUDE_BUILD_DIR}/CL
     INSTALL_COMMAND   ""
@@ -118,10 +131,15 @@ if( NOT OpenCL_LIBRARIES )
       list(APPEND AUX_CMAKE_FLAGS -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER})
   endif()
   file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/icd_build)
+  set(OpenCL_ICD_LOADER_SOURCE_DIR "${CMAKE_CURRENT_BINARY_DIR}/OpenCL/icd")
+  if (NOT DEFINED SYCL_EP_OCL_ICD_SKIP_AUTO_UPDATE)
+    set(SYCL_EP_OCL_ICD_SKIP_AUTO_UPDATE ${SYCL_EXTERNAL_PROJECTS_SKIP_AUTO_UPDATE})
+  endif()
   ExternalProject_Add(ocl-icd
     GIT_REPOSITORY    https://github.com/KhronosGroup/OpenCL-ICD-Loader.git
     GIT_TAG           origin/master
-    SOURCE_DIR        "${CMAKE_CURRENT_BINARY_DIR}/OpenCL/icd"
+    UPDATE_DISCONNECTED ${SYCL_EP_OCL_ICD_SKIP_AUTO_UPDATE}
+    SOURCE_DIR        ${OpenCL_ICD_LOADER_SOURCE_DIR}
     BINARY_DIR        "${CMAKE_CURRENT_BINARY_DIR}/icd_build"
     INSTALL_DIR       "${CMAKE_CURRENT_BINARY_DIR}/icd_install"
     CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}

sycl/plugins/level_zero/CMakeLists.txt

@@ -17,10 +17,15 @@ if (NOT DEFINED L0_LIBRARY OR NOT DEFINED L0_INCLUDE_DIR)
       list(APPEND AUX_CMAKE_FLAGS -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER})
   endif()
   file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/l0_loader_build)
+  set(L0_LOADER_SOURCE_DIR "${CMAKE_CURRENT_BINARY_DIR}/Level0/l0_loader")
+  if (NOT DEFINED SYCL_EP_L0_LOADER_SKIP_AUTO_UPDATE)
+    set(SYCL_EP_L0_LOADER_SKIP_AUTO_UPDATE ${SYCL_EXTERNAL_PROJECTS_SKIP_AUTO_UPDATE})
+  endif()
   ExternalProject_Add(l0-loader
     GIT_REPOSITORY    https://github.com/oneapi-src/level-zero.git
     GIT_TAG           origin/master
-    SOURCE_DIR        "${CMAKE_CURRENT_BINARY_DIR}/Level0/l0_loader"
+    UPDATE_DISCONNECTED ${SYCL_EP_L0_LOADER_SKIP_AUTO_UPDATE}
+    SOURCE_DIR        ${L0_LOADER_SOURCE_DIR}
     BINARY_DIR        "${CMAKE_CURRENT_BINARY_DIR}/l0_loader_build"
     INSTALL_DIR       "${CMAKE_CURRENT_BINARY_DIR}/l0_loader_install"
     CMAKE_ARGS -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}