CMake: Enable shared builds

[Croydon]

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:

enet/include/enet/win32.h

Line 40 in 276ff5a

#ifdef ENET_DLL

And this is only available with MSVC and with Clang when compiling for Windows.

---

• This fixes Add shared library link option? #2.
• This pull request, together with the merged changes of CMake: Fix linking for Windows builds with non-MinGW compilers #241 and Future proof cmake minimum version #187, also makes CMake improvements #147 obsolete. Closing CMake improvements #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 CMake: Fix linking for Windows builds with non-MinGW compilers #241). Fixing compiling a dll in visual studio #106.
• Edit: Oops, I used wrong keywords. closing vs closes and fixing vs fixes.... will add it to my next pull request again.