This change in redhat-rpm-config:

* Wed Jun 03 2020 Igor Raits ignatenkobrain@fedoraproject.org - 158-1 - Add option to choose C/C++ toolchain

changed %_set_build_flags to also set the CC/CXX compiler variables:

CC=%{__cc}; export CC ; \ CXX=%{__cxx}; export CXX ; \

This breaks the relatively common construct for building MPI versions of packages:

export CC=mpicc export CXX=mpicxx %configure

For %configure you can change to:

%configure CC=mpicc CXX=mpicxx

For %cmake (this will also work with %configure) you'll need to do:

%define __cc mpicc %define __cxx mpicxx %cmake

With koschei down (I presume it is due to the move) I think this has gone mostly unnoticed so far.

HTH, Orion