CMake FindPythonInterp deprecated

[pdv]

find_package(PythonInterp ...) is deprecated since CMake 3.12.

With the current master build on MacOS CMake returns 
`Python3_EXECUTABLE=Python3.5` although Python3.7 is selected with macports.

Using the recommended find_package(Python3 ...) CMake returns 
`Python3_EXECUTABLE=Python3.7`. (patch included)

Patrick
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diff --git a/CMakeLists.txt b/CMakeLists.txt
index 779d9e932d..5b8b215bb2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -786,22 +786,26 @@ if(GNUWIN32_DIR)
 	list(APPEND CMAKE_PROGRAM_PATH "${GNUWIN32_DIR}/Python" )
 endif()
 
-# Search for python default version first
-unset(PYTHON_EXECUTABLE CACHE)
-unset(LYX_PYTHON_EXECUTABLE CACHE)
-unset(PYTHON_VERSION_MAJOR)
-unset(PYTHON_VERSION_MINOR)
-unset(PYTHON_VERSION_STRING)
-find_package(PythonInterp 3.5 QUIET)
-if(NOT PYTHONINTERP_FOUND)
-  unset(PYTHON_EXECUTABLE CACHE)
-  find_package(PythonInterp 2.0 REQUIRED)
-  if(NOT PYTHON_VERSION_STRING VERSION_LESS 2.8)
-    message(FATAL_ERROR "Python interpreter found, but is not suitable")
-  endif()
-endif()
+# Search for python default version if not yet defined
+if(NOT DEFINED LYX_PYTHON_EXECUTABLE)
+	unset(PYTHON_EXECUTABLE CACHE)
+	unset(LYX_PYTHON_EXECUTABLE CACHE)
+	unset(PYTHON_VERSION_MAJOR)
+	unset(PYTHON_VERSION_MINOR)
+	unset(PYTHON_VERSION_STRING)
+	find_package(Python3 3.5 QUIET)
+	if(NOT Python3_Interpreter_FOUND)
+	  unset(PYTHON_EXECUTABLE CACHE)
+	  find_package(Python2 2.0 QUIET)
+	  if(NOT PYTHON_VERSION_STRING VERSION_LESS 2.8)
+	    message(FATAL_ERROR "Python interpreter found, but is not suitable")
+	  else()
+	   set(LYX_PYTHON_EXECUTABLE ${Python2_EXECUTABLE} CACHE FILEPATH "Python to be used by LyX")
+	  endif()
+	endif()
 
-set(LYX_PYTHON_EXECUTABLE ${PYTHON_EXECUTABLE} CACHE FILEPATH "Python to be used by LyX")
+	set(LYX_PYTHON_EXECUTABLE ${Python3_EXECUTABLE} CACHE FILEPATH "Python to be used by LyX")
+endif()
 
 if(LYX_NLS)
 	find_package(LyXGettext)