Fix two CMake uninitialized-variable warnings

[ferdnyc]

• Ensure GAMMARAY_PROBE_ABI_POSTFIX is always set, even if it's empty.
• Use QT_VERSION_MAJOR in plugins/qmlsupport instead of QT_MAJOR_VERSION, to match rest of the CMake code.

[dantti]

Since we now require Qt 5.15 we could instead just use versionless targets

[winterz]

I'm merging this as it is a proper fix.
we can change to versionless if we want in a new PR