FYI I just hit the same issue on latest root master on my Fedora 35.
I first tried rm lib/*.pcm as @Axel suggested. Then cmaking however failed with a similar error - in particular the old error plus:
gmake[2]: *** [sql/mysql/CMakeFiles/G__RMySQL.dir/build.make:88: sql/mysql/G__RMySQL.cxx] Error 1
gmake[2]: *** [io/sql/CMakeFiles/G__SQLIO.dir/build.make:91: io/sql/G__SQLIO.cxx] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:41433: sql/mysql/CMakeFiles/G__RMySQL.dir/all] Error 2
gmake[1]: *** Waiting for unfinished jobs....
gmake[1]: *** [CMakeFiles/Makefile2:35775: io/sql/CMakeFiles/G__SQLIO.dir/all] Error 2
[ 75%] Built target unordered_mapDict
<<< cling interactive line includer >>>: remark: building module 'MultiProc' as '[...]/rb/lib/MultiProc.pcm' [-Rmodule-build]
Error: Building module 'MultiProc' implicitly. If 'MultiProc' requires a
dictionary please specify build dependency: 'G__Imt.cxx' depends on 'MultiProc'.
Otherwise, specify '-mByproduct MultiProc' to disable this diagnostic.
Subprocess aborted
gmake[2]: *** [net/auth/CMakeFiles/G__RootAuth.dir/build.make:89: net/auth/G__RootAuth.cxx] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:36142: net/auth/CMakeFiles/G__RootAuth.dir/all] Error 2
Subprocess aborted
Then, I applyed @eguiraud 's suggestion to delete all .pcm files (i.e. find . -name "*.pcm" -type f -delete), then cmake was successful.
I never changed the number of cores in cmake - always entering -j32.