Recommend pkg-config when systemd fails

We used to show an error like this when configure failed to find systemd:

configure: error: Package requirements (systemd >= 209) were not met:
No package 'systemd' found
Consider adjusting the PKG_CONFIG_PATH environment variable if you installed software in a non-standard prefix.
Alternatively, you may set the environment variables SYSTEMD209_CFLAGS and SYSTEMD209_LIBS to avoid the need to call pkg-config. See the pkg-config man page for more details.

But now we don't:

checking for pkg-config... no
checking for SYSTEMD... no
configure: Okay, checking for systemd a different way...
checking for SYSTEMD... no
configure: error: Explicitly requested systemd support, but systemd not found

This issue also affects lzma, and any other package that requires pkg-config.

In fact, lzma silently fails:

./configure --enable-lzma silently fails if pkg-config is not installed: ​ https://gitlab.com/eighthave/tor/pipelines/85811623