fatal error: absl/types/optional.h: No such file or directory

[FrontierDK]

Having made the generate_scorer_package package before, I have written a text file from which I have copy/pasted before, with success, when the 1.4.0 came out.

However, when I now try to run the exact same list in a new fresh Ubuntu 20.04.05, I get this error when running the make generate_scorer_package

c++     -Wl,--no-as-needed -Wl,-rpath,\$ORIGIN -L/home/me/STT/tensorflow/bazel-bin/native_client -L/home/me/STT/tensorflow/bazel-bin/tensorflow/lite  -lstt -lkenlm -llzma -lbz2   generate_scorer_package.cpp   -o generate_scorer_package
generate_scorer_package.cpp:8:10: fatal error: absl/types/optional.h: No such file or directory
    8 | #include "absl/types/optional.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [<builtin>: generate_scorer_package] Error 1

How does one fix this error? It doesn't seem to matter which branch I clone...