wip: Add scikit-build-classic FindCython/UseCython

Based of scikit-build/scikit-build@c723e579a (fix: stop assuming that pyx
files are in the same directory as CMakeLists.txt)

Author: jcfr

projects/core-cython-hello/cmake/FindCython.cmake

@@ -0,0 +1,88 @@
+#.rst:
+#
+# Find ``cython`` executable.
+#
+# This module will set the following variables in your project:
+#
+#  ``CYTHON_EXECUTABLE``
+#    path to the ``cython`` program
+#
+#  ``CYTHON_VERSION``
+#    version of ``cython``
+#
+#  ``CYTHON_FOUND``
+#    true if the program was found
+#
+# For more information on the Cython project, see https://cython.org/.
+#
+# *Cython is a language that makes writing C extensions for the Python language
+# as easy as Python itself.*
+#
+#=============================================================================
+# Copyright 2011 Kitware, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#=============================================================================
+
+# Use the Cython executable that lives next to the Python executable
+# if it is a local installation.
+if(Python_EXECUTABLE)
+  get_filename_component(_python_path ${Python_EXECUTABLE} PATH)
+elseif(Python3_EXECUTABLE)
+  get_filename_component(_python_path ${Python3_EXECUTABLE} PATH)
+elseif(DEFINED PYTHON_EXECUTABLE)
+  get_filename_component(_python_path ${PYTHON_EXECUTABLE} PATH)
+endif()
+
+if(DEFINED _python_path)
+  find_program(CYTHON_EXECUTABLE
+               NAMES cython cython.bat cython3
+               HINTS ${_python_path}
+               DOC "path to the cython executable")
+else()
+  find_program(CYTHON_EXECUTABLE
+               NAMES cython cython.bat cython3
+               DOC "path to the cython executable")
+endif()
+
+if(CYTHON_EXECUTABLE)
+  set(CYTHON_version_command ${CYTHON_EXECUTABLE} --version)
+
+  execute_process(COMMAND ${CYTHON_version_command}
+                  OUTPUT_VARIABLE CYTHON_version_output
+                  ERROR_VARIABLE CYTHON_version_error
+                  RESULT_VARIABLE CYTHON_version_result
+                  OUTPUT_STRIP_TRAILING_WHITESPACE
+                  ERROR_STRIP_TRAILING_WHITESPACE)
+
+  if(NOT ${CYTHON_version_result} EQUAL 0)
+    set(_error_msg "Command \"${CYTHON_version_command}\" failed with")
+    set(_error_msg "${_error_msg} output:\n${CYTHON_version_error}")
+    message(SEND_ERROR "${_error_msg}")
+  else()
+    if("${CYTHON_version_output}" MATCHES "^[Cc]ython version ([^,]+)")
+      set(CYTHON_VERSION "${CMAKE_MATCH_1}")
+    else()
+      if("${CYTHON_version_error}" MATCHES "^[Cc]ython version ([^,]+)")
+        set(CYTHON_VERSION "${CMAKE_MATCH_1}")
+      endif()
+    endif()
+  endif()
+endif()
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(Cython REQUIRED_VARS CYTHON_EXECUTABLE)
+
+mark_as_advanced(CYTHON_EXECUTABLE)
+
+include(UseCython)