fix build failures when using shared libraries

When building shared libraries as opposed to static libraries, a variety of
build and link errors would be encountered. This all stems from commit
53344ec8. The addition of `hidden` visibility in
some circumstances appears flat out wrong, in that it contradicts both GCC
advice¹ as well as other guides to symbol visibility.²

The corrected solution applied in this commit is to attach *no* visibility when
compiling something that links against librumur, letting the default (possibly
set through `-fvisibility=…`) take effect.

¹ https://gcc.gnu.org/wiki/Visibility
² https://www.gnu.org/software/gnulib/manual/html_node/Exported-Symbols-of-Shared-Libraries.html

Github: fixes #220 “2021.08.28: Fails to link: error: undefined hidden…”