Manually prepend `lib` and append `.a` to the static library names
provided by the cmake pkg-config cmake module. This is necessary because
the `find_library()` function always prefers the shared object file if
the library name is "unqualified" and unfortunately the cmake pkg-config
module seems to provide only "unqualified" library names through
`*_PKG_STATIC_LIBIRARIES`.
Enclose option() declaration in a defined check to allow simple caller
set() use case. Provide default static lib name for find_library since
pkgconfig for libsodium currently does not provide it.
