[py3] Teach the CMake build to reject Python versions older than 2.7.

Continue to require Python 2 however as recent experiments suggest
LLDB's build requires it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224948 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -344,7 +344,9 @@ set(TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")
 include(HandleLLVMOptions)
 
 # Verify that we can find a Python 2 interpreter.  Python 3 is unsupported.
-set(Python_ADDITIONAL_VERSIONS 2.7 2.6 2.5)
+# FIXME: We should support systems with only Python 3, but that requires work
+# on LLDB.
+set(Python_ADDITIONAL_VERSIONS 2.7)
 include(FindPythonInterp)
 if( NOT PYTHONINTERP_FOUND )
   message(FATAL_ERROR
@@ -353,6 +355,10 @@ if( NOT PYTHONINTERP_FOUND )
 Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
 endif()
 
+if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
+  message(FATAL_ERROR "Python 2.7 or newer is required")
+endif()
+
 ######
 # LLVMBuild Integration
 #