Q_CC_GNU is defined on compilers that masquerade as GCC (Clang, ICC),
so using it to work around GCC-specific bugs is wrong. Introduce a
local define for _only_ GCC and use it in place of Q_CC_GNU.
Drive by: version-fence a test we now know it's been fixed upstream,
and correct the link to the corresponding bug report.
Pick-to: 5.15 6.0 6.1
Change-Id: I9059d6e6bf86157aca71590ac22afb1a1c114313
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
8b9c8d417b