message: Patch from Dmitry : Enforce cross build restrictions.
diff: @@ -70,7 +70,10 @@ if (ISPC_CROSS)
  option(ISPC_IOS_TARGET "Build ISPC with ios target support" ON)
  option(ISPC_ANDROID_TARGET "Build ISPC with android target support" ON)
  option(ISPC_PS4_TARGET "Build ISPC with ps4 target support" ON)
  + # We silently disable some of cross targets. We should probably warn user or issue an error,
  + # but it seems good enough.
  if (WIN32)
  + set(ISPC_IOS_TARGET OFF)
  if ((ISPC_LINUX_TARGET OR ISPC_ANDROID_TARGET OR ISPC_PS4_TARGET) AND NOT ISPC_GNUWIN32_PATH)
  message (FATAL_ERROR "Set ISPC_GNUWIN32_PATH variable for cross compilation to Linux/Android/PS4 e.g. C:/gnuwin32")
  endif()
  @@ -78,6 +81,8 @@ if (ISPC_CROSS)
  message (FATAL_ERROR "Set ISPC_MACOS_SDK_PATH variable for cross compilation to MacOS e.g. C:/iusers/MacOSX10.14.sdk")
  endif()
  elseif (APPLE)
  + set(ISPC_WINDOWS_TARGET OFF)
  + set(ISPC_PS4_TARGET OFF)
  if ((ISPC_LINUX_TARGET OR ISPC_ANDROID_TARGET) AND NOT ISPC_ANDROID_NDK_PATH)
  message (FATAL_ERROR "Set ISPC_ANDROID_NDK_PATH variable for cross compilation to Linux/Android e.g. /Users/Shared/android-ndk-r20")
  endif()
  @@ -91,6 +96,9 @@ if (ISPC_CROSS)
  message(STATUS "Using iOS SDK path ${ISPC_IOS_SDK_PATH}")
  endif()
  else()
  + set(ISPC_WINDOWS_TARGET OFF)
  + set(ISPC_PS4_TARGET OFF)
  + set(ISPC_IOS_TARGET OFF)
  if (ISPC_MACOS_TARGET AND NOT ISPC_MACOS_SDK_PATH)
  message (FATAL_ERROR "Set ISPC_MACOS_SDK_PATH variable for cross compilation to MacOS e.g. /iusers/MacOSX10.14.sdk")
  endif()
  