We used the following construct to read the version string from a file called VERSION:

if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/VERSION")
  file(READ "${CMAKE_CURRENT_SOURCE_DIR}/VERSION" PROGRAM_VERSION)
  string(STRIP "${PROGRAM_VERSION}" PROGRAM_VERSION)

else()
  message(FATAL_ERROR "File ${CMAKE_CURRENT_SOURCE_DIR}/VERSION not found")
endif()

Here, we first check that this file exists, and issue an error message if it does not. If it exists, we read its contents into a variable called PROGRAM_VERSION, which we strip of any trailing whitespace. Once the variable PROGRAM_VERSION is set, it can be used to configure version.hpp.in to generate generated/version.hpp, as follows:

configure_file(
  version.hpp.in
  generated/version.hpp
  @ONLY
  )