Re-enable C++14 for GCC 4.9 in Interpretator

GCC 4.9 only defines '__cplusplus' to the value 201300L and not to
201402L as expected for C++14. This makes the check for the available
C++ standards in the interpreter fail to enable C++14 (which in turn
means that ROOT cannot be compiled with GCC 4.9 if C++14 was enabled in
Cmake). Similarly, for C++17 apart from the proper value 201703L other
values seems to be floating around (e.g. 201406L as defined by the
version of LLVM included with ROOT). The requirement for '__cplusplus_'
to enable certain C++ standards in the interpreter is made less strict
and just needs to be larger than the previous final value.

This basically reinstates commit
0a62e34aa86b812651cfcf9526ba03b975adaa5c which was undone by commit
702298d9ad83866d0be62f0422c03ac8ea6687f1.

lib/Interpreter/CIFactory.cpp

@@ -399,10 +399,10 @@ namespace {
     // I would claim that the check should be relaxed to:
 
     if (Opts.CPlusPlus) {
-#if __cplusplus >= 201703L
+#if __cplusplus > 201402L
       Opts.CPlusPlus1z = 1;
 #endif
-#if __cplusplus >= 201402L
+#if __cplusplus > 201103L
       Opts.CPlusPlus14 = 1;
 #endif
 #if __cplusplus >= 201103L