Clean up comment about uid variable

Fix libcurl version check

Fix CPR variable name

After trying to use CPR 1.10.0 first, which uses a new variable name, we had to move back to 1.8.3, which works well with the system libcurl on Ubuntu bionic.

Remove unnecessary include

Breaks build.

Add support for SSL_CERT_FILE environment variable

This environment variable is supported by the CLI curl tool, too. It allows using a custom CA bundle with the tool.

Reasons to use this environment variable include the use of self-signed or otherwise untrusted certificates on the server or setting a CA bundle path when using the tool on a distro whose path differs from the one used on the build system.

Properly exclude tests when BUILD_TESTING is OFF

Update cpr

Allow using system-wide installed args library

Fix x86_64 build