message: Fail on CMake generation if re2c not found
diff: @@ -35,7 +35,11 @@ find_package(Z3 4.8.5 REQUIRED)
  include_directories(${Z3_INCLUDE_DIR})
  find_program(RE2C re2c)
  +if (RE2C)
  message(STATUS "RE2C: ${RE2C}")
  +else()
  + message(SEND_ERROR "re2c executable not found")
  +endif()
  add_custom_command(OUTPUT "${CMAKE_BINARY_DIR}/tools/alive_lexer.cpp"
  COMMAND ${CMAKE_COMMAND} -E make_directory "${CMAKE_BINARY_DIR}/tools"
  COMMAND ${RE2C} ARGS "-d" "-b" "-T" "--no-generation-date"
  