To compile with 64bit labels set

WM_LABEL_SIZE=64

in ~/OpenFOAM/dev/prefs.sh

source ~/.bashrc

then Allwmake in OpenFOAM-dev.

This will build into for example OpenFOAM-dev/platforms/linux64ClangDPInt64Opt

If WM_LABEL_SIZE is unset or set to 32:

WM_LABEL_SIZE=32

the build would be placed into OpenFOAM-dev/platforms/linux64ClangDPInt32Opt

Thus both 32bit and 64bit label builds can coexist without problem.