Most free operating system packagers get pretty shirty when you embed a copy of something that they already ship. They'd rather have all libraries be shared libraries, and have only one copy of each one exist. This lets them apply patches as needed in only one place, lets them upgrade every copy on the system at once, and so on.
In order to get libevhtp usable by programs that want to get shipped by (say) debian or fedora, there is going to need to be a documented option for how to build with a system copy of oniguruma. (Ideally, it could just detect an oniguruma of the appropriate version or later, and not even bug the user.)
$ sudo apt-get install libonig2 libonig-dev
$ dpkg -L libonig-dev|grep h$
$ dpkg -L libonig2|grep \.so
With some patching this can be done. See for example the Debian packaging in https://github.com/dvandok/libevhtp
this was done a very long time ago. It checks to see if it's already installed. going to close this ticket.