Add CMake option to disable installation of headers and libraries

[kodemeister]

This PR is intended to fix LizardByte/Sunshine#1591. Basically Sunshine uses CMake's add_subdirectory to add enet library to the project. Unfortunately this causes enet headers and static library to be installed into Sunshine prefix. All Debian, Ubuntu, Fedora, Arch (and probably other) nightly builds contain the following unwanted files:

• /usr/include/enet/*.h
• /usr/lib/static/libenet.a

The PR adds new CMake option named ENET_NO_INSTALL to disable installation of any enet files. By default installation is enabled. If changes look good to you, I'll create another PR in Sunshine repo which will use ENET_NO_INSTALL to get rid of enet headers/libs.

In fact, Sunshine already does something similar for miniupnpc library. It uses UPNPC_NO_INSTALL option to disable installation of its headers/libs:

set(UPNPC_NO_INSTALL ON CACHE BOOL "Don't install any libraries build for miniupnpc")

[cgutman]

Thanks, merged and bumped the ENet submodule in moonlight-common-c so Sunshine can pick up the change.

[kodemeister]

Perfect, thank you! Will make a companion PR to Sunshine repo with that bumped submodule.