cmake: Add support for CMAKE_CROSSCOMPILING_EMULATOR

This commit updates the build systems to allow cross-compiling VTK
using toolchain defining CMAKE_CROSSCOMPILING_EMULATOR.

It simplifies the cross-compilation removing the need to independently
compile the "VTKCompileTools".

See https://cmake.org/cmake/help/v3.8/variable/CMAKE_CROSSCOMPILING_EMULATOR.html#variable:CMAKE_CROSSCOMPILING_EMULATOR

CMake/vtkModuleMacros.cmake

@@ -517,7 +517,7 @@ endfunction()
 # rules for the target unless NO_INSTALL argument is specified or
 # VTK_INSTALL_NO_DEVELOPMENT variable is set.
 function(vtk_compile_tools_target _name)
-  if (CMAKE_CROSSCOMPILING)
+  if (CMAKE_CROSSCOMPILING AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR)
     message(AUTHOR_WARNING
       "vtk_compile_tools_target is being called when CMAKE_CROSSCOMPILING is true. "
       "This generally signifies a script issue. compile-tools are not expected "

CMakeLists.txt

@@ -248,8 +248,9 @@ if(NOT VTK_BUILD_PYTHON_MODULE_DIR)
        PATH "Directory where python modules will be put inside the build tree")
   mark_as_advanced(VTK_BUILD_PYTHON_MODULE_DIR)
 endif()
-if (CMAKE_CROSSCOMPILING AND NOT COMPILE_TOOLS_IMPORTED)
-  # if CMAKE_CROSSCOMPILING is true, we need to import build-tools targets.
+if (CMAKE_CROSSCOMPILING AND NOT COMPILE_TOOLS_IMPORTED AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR)
+  # if CMAKE_CROSSCOMPILING is true and CMAKE_CROSSCOMPILING_EMULATOR is not set, we need
+  # to import build-tools targets.
   find_package(VTKCompileTools REQUIRED)
   set (COMPILE_TOOLS_IMPORTED TRUE)
 endif()

Utilities/EncodeString/CMakeLists.txt

@@ -10,7 +10,7 @@
 set(VTK_BUILD_FORWARDING_EXECUTABLES FALSE)
 vtk_module_export_info()
 
-IF(NOT CMAKE_CROSSCOMPILING)
+IF(NOT CMAKE_CROSSCOMPILING OR DEFINED CMAKE_CROSSCOMPILING_EMULATOR)
   VTK_ADD_EXECUTABLE(vtkEncodeString vtkEncodeString.cxx)
   vtk_compile_tools_target(vtkEncodeString)
 ENDIF()

Utilities/HashSource/CMakeLists.txt

@@ -12,7 +12,7 @@ vtk_module_export_info()
 # launcher for it and should always use the RPATH to find shared libs.
 SET(CMAKE_SKIP_RPATH 0)
 
-IF(NOT CMAKE_CROSSCOMPILING)
+IF(NOT CMAKE_CROSSCOMPILING OR DEFINED CMAKE_CROSSCOMPILING_EMULATOR)
   ADD_EXECUTABLE(vtkHashSource vtkHashSource.cxx)
   TARGET_LINK_LIBRARIES(vtkHashSource vtksys)
   vtk_compile_tools_target(vtkHashSource)

Utilities/ParseOGLExt/CMakeLists.txt

@@ -23,7 +23,7 @@
 
 vtk_module_export_info()
 
-IF(NOT CMAKE_CROSSCOMPILING)
+IF(NOT CMAKE_CROSSCOMPILING OR DEFINED CMAKE_CROSSCOMPILING_EMULATOR)
   ADD_EXECUTABLE(vtkParseOGLExt Tokenizer.cxx ParseOGLExt.cxx)
   vtk_compile_tools_target(vtkParseOGLExt)
 ENDIF()