CMake: Enable shared builds

Currently, the CMake file hard-configured a static build of enet. These are the minimal changes required to enable shared builds of enet as well.

CMake has the default BUILD_SHARED_LIBS option to determinate if a shared or static build is desired.

For builds that

are shared
and target Windows
and are either compiled with MSVC or Clang

the following two definitions are declared:
ENET_DLL (see documentation)
ENET_BUILDING_LIB

Why only for MSVC and Clang? Because enet then makes use of __declspec here: https://github.com/lsalzman/enet/blob/276ff5ae05a245bcee195af9c9fd002851517d40/include/enet/win32.h#L40 And this is only available with MSVC and with Clang when compiling for Windows.

This fixes #2.
This pull request, together with the merged changes of #241 and #187, also makes #147 obsolete. Closing #147.
Visual Studio has official, first-class support for CMake projects. Therefore allowing easily shared enet builds with VS when these changes are applied (together with the previously merged #241). Fixing #106.
Edit: Oops, I used wrong keywords. closing vs closes and fixing vs fixes.... will add it to my next pull request again.

lsalzman / enet