Allow build with systems libenet, but still support own version.

[susnux]

This will not completely unbundle enet like https://github.com/acreloaded/acr/pull/12 But will support build with a enet version provided by the system.

So like said in https://github.com/acreloaded/acr/pull/195 some Linux distributions do not provide enet, that distros would then use the bundled one. But other can use their system one.

Tested it with current release and enet7 (1.3.13) works :-)

[theonlypwner]

I will probably merge this in the future, but there is another consideration:

If enet updates their protocol, developers might accidentally compile an incompatible version.

[susnux]

@theonlypwner Maybe adding a switch like "ENABLE_SYSTEM_ENET" ? which has to be set to yes? So it can not be done accidentally but packagers or developers can use it if they know what they are doing?

[theonlypwner]

Adding a switch is a good idea.

[Mte90]

I compiled on debian with makefile without no errors :-)