pkgconfig: set libdir only if used
pkgconfig: set libdir only if used

This should address some concerns outlined in
https://github.com/yhirose/cpp-httplib/pull/1182

pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson