CXX-2832 Restore VERSION_CURRENT in release tarball

[kevinAlbs]

Summary

Restore the VERSION_CURRENT file in the tarball produced by the dist target.

Background & Motivation

The 3.8.1 release tarball includes the file build/VERSION_CURRENT. The 3.9.0 release tarball (currently) does not include a build/VERSION_CURRENT file. This requires users to specify the version when building.

The missing VERSION_CURRENT file was discovered investigating CDRIVER-4834. It appears to be the result of changes in #1020.

Building the dist target on b133ac5 results in the VERSION_CURRENT file being omitted:

#!/usr/bin/env bash

# Clear build directory
rm -rf build/*
cd build

# Make dist tarball similar to the release process.
echo "3.9.0-pre" > VERSION_CURRENT
cmake \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_PREFIX_PATH=/Users/kevin.albertson/code/c-bootstrap/install/mongo-c-driver-1.25.1 \
    -DENABLE_UNINSTALL=ON \
    ..
cmake --build . --target dist

# Untar dist tarball.
tar xf mongo-cxx-driver-r3.9.0-pre.tar.gz

# Check for VERSION_CURRENT
if [[ -f mongo-cxx-driver-r3.9.0-pre/build/VERSION_CURRENT ]]; then
    echo "VERSION_CURRENT found with contents $(cat mongo-cxx-driver-r3.9.0-pre/build/VERSION_CURRENT) Test passed"
else
    echo "VERSION_CURRENT not found. Test failed"
    exit 1
fi

# Test passes on 3.8.1. Test fails on commit b133ac58.

This PR changes the path to VERSION_CURRENT from absolute to relative to address this error:

make[3]: *** No rule to make target `Users/kevin.albertson/code/tasks/mongo-cxx-driver-restore-VERSION_CURRENT/build/VERSION_CURRENT', needed by `mongo-cxx-driver-r3.9.0-pre.tar.gz'.  Stop.`

The distcheck target has been updated to ensure VERSION_CURRENT is present.

Once this PR is merged, I plan to update the 3.9.0 release tarball to include the VERSION_CURRENT file.

[kevinAlbs]

Filed CXX-2834 to propose including a VERSION_CURRENT file in the source. That may help prevent the possibility of building a tarball without a VERSION_CURRENT file.

[kevinAlbs]

Tarball on https://github.com/mongodb/mongo-cxx-driver/releases/tag/r3.9.0 has been updated.