Fix compilation with gcc 4.9

It appears that gcc 4.9 does not implement the following part of C++11:
https://cplusplus.github.io/CWG/issues/1148.html

Therefore, we have to use a special case in C++11 mode that does an
explicit std:move.

With recent compilers (gcc >= 9), this leads in C++11 mode to a warning:

MetricsInfo.cpp: In member function ‘lyx::Changer lyx::MetricsBase::changeFontSet(const string&)’:
../../master/src/MetricsInfo.cpp:83:13: warning: redundant move in return statement [-Wredundant-move]
   83 |  return move(rc);
      |         ~~~~^~~~
MetricsInfo.cpp:83:13: note: remove ‘std::move’ call

Partly reverts commit fff28c57.

src/MetricsInfo.cpp

@@ -75,7 +75,13 @@ Changer MetricsBase::changeFontSet(string const & name)
 	    && ((isTextFont(oldname) && oldcolor != Color_foreground)
 	        || (isMathFont(oldname) && oldcolor != Color_math)))
 		font.setColor(oldcolor);
+#if __cplusplus >= 201402L
 	return rc;
+#else
+	/** In theory, this is not needed with C++11, and modern compilers
+	 * will complain in C++11 mode, but gcc 4.9 requires this. */
+	return std::move(rc);
+#endif
 }