Re: [Libguestfs] [nbdkit PATCH v4 1/4] server: Export nbdkit_set_dlopen_prefix function

At least the VDDK plugin needs a way to load .so files with a custom directory prepended. Asking the user to set LD_LIBRARY_PATH is too strong (that would leak into the child process created by --run, and makes the user think about issues that we'd rather cover automatically). But overriding dlopen() to make it easier to prepend a directory name under our control requires either a separate namespace via dlmopen() (which itself is annoying - it messes up gdb debugging), or that the override occur prior to any other library that also depends on -ldl. Which means that if we are going to provide this functionality, we have to do so from nbdkit proper. Note that properly injecting a dlopen shim for all subsequent shared library loads requires that the override itself be in a shared library. Thus, nbdkit has to pull it in via a shared library link, rather than merely a .o file, and we now have to install something in pkglibdir. What's more, now that we link against a shared library instead of just libtool convenience libraries, libtool now creates a wrapper app named lt-nbdkit so that it can run an in-tree build that still locates the uninstalled shared library; this affects some expected output in tests.