Undefined symbol __extendhfsf2 libQt6Core.so.6.5.0

Build using gcc 12 on x86 Solaris fails with:

[614/2696] Linking CXX shared library qtbase/lib/amd64/libQt6Core.so.6.5.0
FAILED: qtbase/lib/amd64/libQt6Core.so.6.5.0

: && /usr/gcc/12/bin/g++ -fPIC -m64 -O3 [...]

Undefined                       first referenced
 symbol                             in file
__extendhfsf2                       qtbase/src/corelib/CMakeFiles/Core.dir/global/qfloat16.cpp.o
__extendhfdf2                       qtbase/src/corelib/CMakeFiles/Core.dir/kernel/qvariant.cpp.o
__truncsfhf2                        qtbase/src/corelib/CMakeFiles/Core.dir/global/qfloat16.cpp.o

Seems that -lgcc should have been included??!