Detecting CXX compiler ABI info - failed

[Franco Comida]

I'm building on copr a development version of luminance-hdr which is in
stable at version 2.4.0.
Build failed for rawhide-i386/x86_64

This bug could be the reason for the failure:

 https://forum.qt.io/topic/69057/where-std-gnu-11-came-from/3

So i changed spec file to conditionally add -fext-numeric-literals needed
by gnu++11, but not build fails only in rawhide-x86_64

https://copr-be.cloud.fedoraproject.org/results/fcomida/LuminanceHDR-SNAP...

The pulling in of -std=gnu++11 in rawhide should be fixed anyway