[cmake] print warning if oms not enabled

[r00t-]

this is the possible learning from #642 : print a warning if oms is not being enabled, just like for sml.

questions:

• this means the warning will be printed for most users, is this OK or possibly confusing?
  • this is the same as for sml, but sml is required by most of our users
  • should oms maybe not be enabled by default? (but that would just lead to repeats of https://github.com/volkszaehler/vzlogger/issues/642 )
• code-wise there's three variables involved (like for sml):
  • ENABLE_*: user inpput to request feature (set by default)
  • *_FOUND: result of library search
  • *_SUPPORT: set conditional if feature can be enabled the sml warning uses ENABLE AND NOT FOUND, i rather use ENABLE AND NOT SUPPORTED to avoid duplicating the decision process. but it's inconsistent... but probably irrelevant.