Fix compilation with Qt <= 5.12

The `__has_feature` macro is a clang extension that works like a macro. If
zint is being compiled using a compiler other then clang, `__has_feature`
is not defined. As it is not defined, it cannot be used as a function call
expression. Any environment that doesn't have `__has_feature` should prevent
the preprocessor from "seeing" the invokation of the macro, as invoking
undefined macros is not supported by the C language.

The usual procedure to assure this would be a construction like
  \#if defined(__has_feature)
  \#  if __has_feature(...)
  \#  endif
  \#endif

which, combined with the GCC check we have here, would result in a
3 level nesting of if and elseif expressions .. and that's without
covering microsoft's compiler.

For this purpose, Qt >= 5.13 has been defining the `__has_feature`
macro on non-clang environments, while defining all (clang) features
to not be available. This allows to write feature checks with "less"
nesting, as we have here.

Mimic Qt-5.13's behavior and provide the `__has_feature` macro if
it's not provided otherwise (either by clang or by Qt), allowing the
function-call-like expression to be parsed on those systems.

backend_qt/tests/test_qzint.cpp

@@ -30,7 +30,10 @@ public:
     {
     // Qt5 will trigger "detected memory leaks" if font used (libfontconfig) so skip if ASAN enabled
 #if QT_VERSION < 0x60000
-#  if defined(__SANITIZE_ADDRESS__) || (defined(__has_feature) && __has_feature(address_sanitizer))
+#  if !defined(__has_feature)
+#    define __has_feature(x) 0
+#  endif
+#  if defined(__SANITIZE_ADDRESS__) || __has_feature(address_sanitizer)
         m_skipIfFontUsed = true;
 #  endif
 #endif