#5 Make it possible to compile without libreplace presence

[Ronald Landheer-Cieslak]

libarch currently doesn't compile without libreplace
because it tries to include libreplace/dirname.h in the
absence of libgen.h. However, libarch doesn't use any
code from libreplace at all, so it should be
unnecessary to have libreplace present when compiling
libarch.

To fix this, we need to check in configure.ac.in
whether libreplace is present and not add -include
libreplace/dirname.h to CFLAGS if it isn't.