Without this commit the error message is:

$ cmake . -DWITH_WSREP=ON
-- Running cmake version 3.12.1
-- MariaDB 10.4.3
CMake Error at cmake/wsrep.cmake:36 (FILE):
  FILE STRINGS file
  ".../mariadb-server-10.4/wsrep-lib/wsrep-API/v26/wsrep_api.h"
  cannot be read.
Call Stack (most recent call first):
  CMakeLists.txt:147 (INCLUDE)

Having 'git config cmake.update-submodules yes' nicely updates the
build without error.

$ cmake . -DWITH_WSREP=ON
-- Running cmake version 3.12.1
-- MariaDB 10.4.3
Submodule path 'wsrep-lib': checked out 'e7d72ae7f6a6995a21d743389426a963429a1fff'
Submodule path 'wsrep-lib/wsrep-API/v26': checked out '75a5f452f2ba07b0f4a3a9a94825fccc71b27398'