Add HTTPS_BACKEND_CURL_LINKED preprocessor AND CMake option.

[MikuAuahDark]

Specifying HTTPS_BACKEND_CURL_LINKED means the libcurl will be linked along with the resulting https shared library, which may link with more binaries than necessary.

Only enable this option if you have policy of prohibiting shared object loading at runtime or if your platform doesn't support it.

Fixes #16

[TurtleP]

Tested this on LÖVE Potion 3.0 Prerelease (took me long enough). It doesn't compile currently, though discussion with @MikuAuahDark has shown there's two needed #ifndef wrappers:

https://github.com/TurtleP/lua-https/blob/linked_curl/src/generic/CurlClient.cpp#L15-L22
https://github.com/TurtleP/lua-https/blob/linked_curl/src/generic/CurlClient.cpp#L34-L45

This will allow it to build an run, but the homebrew consoles I run this on do not come with any kind of CA Certificates. I could either embed an extracted one for all platforms or (and likely not preferred) disable CURLOPT_SSL_VERIFYPEER. Because of this issue, all result codes return zero.

EDIT:
I forgot to mention, main.cpp will need renaming because, well, typically the main application this will embed into will already have a main.cpp. Because of this, maybe rename it to main_lua.cpp, lua_https.cpp, or whatever would not cause a likely name conflict during compilation.

[MikuAuahDark]

Closed in favor of #30.