Fix build with Qt 4

Summary:
In Qt 4, the compiler detection macros have no value, so they can't be compared
against. So just use the compiler-specific ones.
It also makes use of the fact that undefined tokens evaluate to 0 in
comparisons.

Test Plan: jriddell built it with GCC 5 and 7 against Qt 4 and 5.

Reviewers: Breeze, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12277