FindPython: fix NumPy detection when Intel MKL library is installed

In an environment where both NumPy and a recent Intel MKL library are
installed, the detection of numpy include directory fails because a
'import numpy' outputs a MKL related warning message on stdout...
(namely "Intel MKL WARNING: Support of Intel(R) Advanced Vector
Extensions (Intel(R) AVX) enabled only processors has been deprecated.
Intel oneAPI Math Kernel Library will use Intel(R) Streaming SIMD
Extensions 4.2 (Intel(R) SSE4.2) instructions instead.")

I've successfully tested the workaround mentioned at
https://github.com/numpy/numpy/issues/23775#issuecomment-1923327310
which consists in setting the MKL_ENABLE_INSTRUCTIONS=SSE4_2 environment
before importing numpy, hence this proposed workaround.

Modules/FindPython/Support.cmake

@@ -3782,6 +3782,14 @@ if ("NumPy" IN_LIST ${_PYTHON_PREFIX}_FIND_COMPONENTS AND ${_PYTHON_PREFIX}_Inte
     endif()
   endif()
 
+  # Workaround Intel MKL library outputting a message in stdout, which cause
+  # incorrect detection of numpy.get_include() and numpy.__version__
+  # See https://github.com/numpy/numpy/issues/23775
+  if(DEFINED ENV{MKL_ENABLE_INSTRUCTIONS})
+    set(_${_PYTHON_PREFIX}_BACKUP_ENV_VAR_MKL_ENABLE_INSTRUCTIONS ENV{MKL_ENABLE_INSTRUCTIONS})
+  endif()
+  set(ENV{MKL_ENABLE_INSTRUCTIONS} "SSE4_2")
+
   if (NOT _${_PYTHON_PREFIX}_NumPy_INCLUDE_DIR)
     execute_process(COMMAND ${${_PYTHON_PREFIX}_INTERPRETER_LAUNCHER} "${_${_PYTHON_PREFIX}_EXECUTABLE}" -c
                             "import sys\ntry: import numpy; sys.stdout.write(numpy.get_include())\nexcept:pass\n"
@@ -3822,6 +3830,14 @@ if ("NumPy" IN_LIST ${_PYTHON_PREFIX}_FIND_COMPONENTS AND ${_PYTHON_PREFIX}_Inte
     set (${_PYTHON_PREFIX}_NumPy_FOUND FALSE)
   endif()
 
+  # Restore previous value of MKL_ENABLE_INSTRUCTIONS
+  if(DEFINED _${_PYTHON_PREFIX}_BACKUP_ENV_VAR_MKL_ENABLE_INSTRUCTIONS)
+    set(ENV{MKL_ENABLE_INSTRUCTIONS} ${_${_PYTHON_PREFIX}_BACKUP_ENV_VAR_MKL_ENABLE_INSTRUCTIONS})
+    unset(_${_PYTHON_PREFIX}_BACKUP_ENV_VAR_MKL_ENABLE_INSTRUCTIONS)
+  else()
+    unset(ENV{MKL_ENABLE_INSTRUCTIONS})
+  endif()
+
   if (${_PYTHON_PREFIX}_NumPy_FOUND)
     unset (_${_PYTHON_PREFIX}_NumPy_REASON_FAILURE CACHE)