This works whether `BUILD_LIBSYSTEMD` is `ON` or the separate build of
libsystemd picked up by pkg-config is static. The Gentoo Linux package
requires the latter.
This change also handles `BUILD_SHARED_LIBS` being `OFF`. In this case,
the libsystemd dependency does need to be included, even though it is
static.
CMake does not allow you to build shared and static libraries
simultaneously (with a single `add_library` call), but this change
declares the libsystemd dependency as `Requires.private` rather than
omitting it entirely. This is in case someone builds and installs both a
static and shared build and then calls `pkg-config --static sdbus-c++`.
The static build needs to be installed first so that the pkg-config
file from the shared build takes precedence.
