Revert "Utils: Remove old compiler support code"

This reverts commit 8a32bcc5e7.

Broke build with MSVC 2015, update 3

Change-Id: Icb59b70fb00ea2a2ecc8d26c8177d21d82aaccf2
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

tests/unit/unittest/smallstring-test.cpp

@@ -31,6 +31,10 @@
 #include <utils/smallstringio.h>
 #include <utils/smallstringvector.h>
 
+#ifndef __cpp_constexpr
+#define constexpr
+#endif
+
 using namespace ::testing;
 
 using Utils::SmallString;