fix: install generated manpages out of tree

[steadytao]

Fixes #846 by installing generated manpages from the source directory when they are not present in an out-of-tree build directory.

Release tarballs include pre-generated manpages. With an out-of-tree build configured using --disable-md2man, make install could install the binaries but skip the manpages because the install rule only checked the build directory.

This keeps the existing build-directory preference then falls back to $(srcdir) for rsync.1, rsync-ssl.1, rsyncd.conf.5 and the optional rrsync.1.

Tested with a v3.4.3 release tarball using:

• ../configure --disable-md2man --with-rrsync
• make
• make install DESTDIR=...

Before this change, the install tree missed the generated manpages. After this change, rsync.1, rsync-ssl.1, rrsync.1 and rsyncd.conf.5 are installed.

[steadytao]

NetBSD failure looks unrelated to this change. It fails during VM package setup before the rsync build/install path reaches the Makefile change whatsoever.