macos apple silicon: We need a libsodium >= 1.0.4

So on macOS Apple Silicon, the libsodium directory is correctly set when running configure, but it still complains:

```
Setting SODIUM_CFLAGS... -I/opt/homebrew/Cellar/libsodium/1.0.20/include
Setting SODIUM_LDLIBS... -L/opt/homebrew/Cellar/libsodium/1.0.20/lib -lsodium

...

*** We need a libsodium >= 1.0.4 (released 2015-06-11).

```

Problem is I have libsodium installed though:

```
% brew info libsodium
==> libsodium: stable 1.0.20 (bottled), HEAD
NaCl networking and cryptography libraryhttps://libsodium.org/
Installed
/opt/homebrew/Cellar/libsodium/1.0.20 (78 files, 904.6KB) *
```