For reasons unclear, EVG patch builds are failing due to `pkg_check_modules` finding a zstd library, yet unable to determine its version. The well-behaving output looks as follows (note the absence of a detected version in the output):

```
– Found PkgConfig: /data/mci/9696ef2e2a0391327e26506a37e505ee/tmp/tmp.GzBmZI3KJ5/pkg-config (found version "2.5.1")
– Checking for module 'libzstd'
– Found libzstd, version
– Detected zstd version but version 0.8.0 required
```

However, this is not the output current observed. Instead CMake currently errors with:

```
CMake Error at src/libmongoc/CMakeLists.txt:141 (elseif):
given arguments:
"VERSION_LESS" "0.8.0"
Unknown arguments specified
```

This appears to be due to one or more of [CMP0012](https://cmake.org/cmake/help/latest/policy/CMP0012.html) and [CMP0054](https://cmake.org/cmake/help/latest/policy/CMP0054.html) (note we've already been setting these policies to NEW via `cmake_minimum_required()`), where the unquoted variable expansion "${ZSTD_VERSION}" is [treated as _no argument_ when empty/unset](https://cmake.org/cmake/help/latest/manual/cmake-language.7.html#unquoted-argument) rather than an empty string, as if:

```cmake

1. Expectation: elseif ("" VERSION_LESS "0.8.0")
2. Actual:
   elseif (VERSION_LESS "0.8.0")
   ```

I believe the proposed change is a worthwhile improvement, independent of diagnosing why a zstd library is suddenly being found without an accompanying version number.