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>

src/libs/utils/smallstring.h

@@ -45,6 +45,19 @@
 #include <utility>
 #include <vector>
 
+#pragma push_macro("constexpr")
+#ifndef __cpp_constexpr
+#define constexpr
+#endif
+
+#pragma push_macro("noexcept")
+#ifdef __clang__
+#define  __cpp_noexcept 201003
+#endif
+#ifndef __cpp_noexcept
+#define noexcept
+#endif
+
 #ifdef UNIT_TESTS
 #define unitttest_public public
 #else
@@ -851,3 +864,6 @@ using SmallString = BasicSmallString<31>;
 using PathString = BasicSmallString<191>;
 
 } // namespace Utils
+
+#pragma pop_macro("noexcept")
+#pragma pop_macro("constexpr")