Add warning about build number, initialize to 5180

[metalefty]

See also: #1392 (comment)

[metalefty]

The warning message will be shown like this.

CMake Warning at CMakeLists.txt:10 (message):
  Setting BUILD_NUMBER to the value less than 5180 will lose compatibility
  with SoftEtherVPN Stable Edition 4.x.  Set to a value greater than or equal
  to 5180 if you are likely to connect from 4.x clients.

  See also:
  https://github.com/SoftEtherVPN/SoftEtherVPN/issues/1392#issuecomment-867348281


-- The C compiler identification is Clang 11.0.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Looking for sys/auxv.h
-- Looking for sys/auxv.h - found
-- Build date: 24/06/2021
-- Build time: 07:51:50
-- Found PkgConfig: /usr/local/bin/pkg-config (found version "1.7.4")
-- Checking for one of the modules 'libsodium'
-- Found Curses: /usr/lib/libcurses.so
-- Found OpenSSL: /usr/local/lib/libcrypto.so (found version "1.1.1k")
-- Found ZLIB: /usr/lib/libz.so (found version "1.2.11")
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- The CXX compiler identification is Clang 11.0.1
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/meta/github/metalefty/SoftEtherVPN