Use MKDYNAMICROOT=no instead of SHLIBINSTALLDIR=/usr/lib

Not only does the former accomplish the same as the latter, it also
avoids the NetBSD Makefiles putting some -rpath's and -L's into the
linking of librumpclient (which is compiled with USE_SHLIBDIR=yes due
to how it's installed in NetBSD), which then wreak havoc for musl.
Use ${CC} instead of cc for tests and warn if !native