1d3178a quieted the g++ build, but made the warnings even
worse with a clang build.
The addition of the -Wno-sfinae-incomplete caused an unrecognized
option warning using recent clang.
Recent clang build with Qt6.1 also caused several warnings
during the processing of the Qt6.1 header files related to
c++26-extensions.
This adds (under Linux) -Wnoc++-26-extensions to the clang++
command line to quiet those warnings and restricts the
-Wno-sfinae-incomplete command line option to the g++ build.
nhmall
15cfe90d62