[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 VZLOGGER V0.8.3 without "oms" protocol #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.

[narc-Ontakac2]

Enabling by default is OK in my opinion.

I will also add a note to the Debian section of the README that oms is not enabled. This is because the library is not packaged.

[r00t-]

just merging this