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

kstyle/breezesplitterproxy.cpp

@@ -38,9 +38,9 @@
 #endif
 #endif
 #ifndef Q_FALLTHROUGH
-#if defined(Q_CC_GNU) && Q_CC_GNU >= 700
+#if __GNUC__ >= 7
 #define Q_FALLTHROUGH() __attribute__((fallthrough))
-#elif defined(Q_CC_CLANG) && Q_CC_CLANG >= 305
+#elif (__clang_major__ > 3) || (__clang_major__ == 3 && __clang_minor__ >= 5)
 #define Q_FALLTHROUGH() [[clang::fallthrough]]
 #else
 #define Q_FALLTHROUGH()