This commit serves 2 purposes:
1. Fixes CMake 3.13 compatibility when using
` -DDEVILUTIONX_SYSTEM_LIBSODIUM=OFF` (fixes#1222).
2. Ensures we do not build targets that are not depended on by us,
by passing `EXCLUDE_FROM_ALL` to `add_subdirectory`.
Adds an option to build libsodium from source instead of using the
system-provided one.
This is useful on systems that do not provide libsodium, or for testing
changes to libsodium along with devilutionx (by specifying
`-DFETCHCONTENT_SOURCE_DIR_LIBSODIUM`).
This also allows us to configure sodium to our liking when building it from source,
such as `SODIUM_MINIMAL`, which reduces the x64 release binary size from 4.3 MiB to 4.1 MiB.
Gleb Mazovetskiy
Anders Jenbo
Benny Franco Dennis