[LyX/master] Cmake build: Prefer using pyton3 over python2

[Kornel Benko]

commit 0f3b1539e88878290a63b6ac3046f077700a8195
Author: Kornel Benko <kor...@lyx.org>
Date:   Mon Apr 24 17:48:59 2017 +0200

    Cmake build: Prefer using pyton3 over python2
---
 CMakeLists.txt |   22 +++++++++++-----------
 1 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index dba0718..da0776e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -729,19 +729,19 @@ if(GNUWIN32_DIR)
        list(APPEND CMAKE_PROGRAM_PATH "${GNUWIN32_DIR}/Python" )
 endif()
 
-find_package(PythonInterp 2.7 QUIET)
-if(PYTHONINTERP_FOUND)
-       if(PYTHON_VERSION_STRING VERSION_GREATER 2.8)
-       unset(PYTHONINTERP_FOUND)
-       endif()
-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 3.3 QUIET)
 if(NOT PYTHONINTERP_FOUND)
   unset(PYTHON_EXECUTABLE CACHE)
-  unset(PYTHON_VERSION_MAJOR)
-  unset(PYTHON_VERSION_MINOR)
-  unset(PYTHON_VERSION_STRING)
-  find_package(PythonInterp 3.3 REQUIRED)
+  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()
 
 set(LYX_PYTHON_EXECUTABLE ${PYTHON_EXECUTABLE} CACHE FILEPATH "Python to be 
used by LyX")