Automatically test for NEON instructions when building python extensions and...

Automatically test for NEON instructions when building python extensions and enable them if available.

Forgot about --yes USE_NEON_INSTRUCTIONS when removing --yes a moment
ago.  It's on by default now so now --yes really is unneeded.  The dlib
policy on this kind of thing going forward is to always have a cmake
script that tests if a feature like this is available and to enable it
automatically when building python extensions.

dlib/cmake_utils/set_compiler_specific_options.cmake

@@ -12,11 +12,15 @@ if (COMMAND pybind11_add_module)
    # For python users, enable SSE4 and AVX if they have these instructions.
    include(${CMAKE_CURRENT_LIST_DIR}/check_if_sse4_instructions_executable_on_host.cmake)
    if (SSE4_IS_AVAILABLE_ON_HOST)
-      set(USE_SSE4_INSTRUCTIONS ON CACHE BOOL "Use SSE4 instructions")
+      set(USE_SSE4_INSTRUCTIONS ON CACHE BOOL "Compile your program with SSE4 instructions")
    endif()
    include(${CMAKE_CURRENT_LIST_DIR}/check_if_avx_instructions_executable_on_host.cmake)
    if (AVX_IS_AVAILABLE_ON_HOST)
-      set(USE_AVX_INSTRUCTIONS ON CACHE BOOL "Use AVX instructions")
+      set(USE_AVX_INSTRUCTIONS ON CACHE BOOL "Compile your program with AVX instructions")
+   endif()
+   include(${CMAKE_CURRENT_LIST_DIR}/check_if_neon_available.cmake)
+   if (ARM_NEON_IS_AVAILABLE)
+      set(USE_NEON_INSTRUCTIONS ON CACHE BOOL "Compile your program with ARM-NEON instructions")
    endif()
 endif()