Fix startup scripts

[MartinNowak]

• use pkg-config if available
• quote variable expansions to allow spaces in paths

[s-ludwig]

Thanks for these robustness improvements! Does the pkg-config fix compilation on a particular platform? I would like to include that in the changelog message if possible.

Unfortunately, Mac is causing some problems. "readlink -f" doesn't work on Mac and also pkg-config does not find libevent on if it has been installed using brew. Maybe it makes sense to change the condition from "does the command pkg-config work?" to "does pkg-config -libs ... succeed?" and use the hard coded linker flags otherwise...

[MartinNowak]

Does the pkg-config fix compilation on a particular platform?

FreeBSD supports parallel installation of libevent and libevent2 (/usr/local/lib/event2).

readlink -f

It's gone, but I still removed the which.

does pkg-config -libs ... succeed

Turns out to be briefer too.

[s-ludwig]

I just had to add libssl as an explicit dependency but now it works everywhere.. and I could have sworn that the "which" was needed, but obviously it's not.