Honor configure arguments --exec-prefix, --bindir, --datarootdir, --mandir

This patch makes the Makefile honor the configure arguments `--exec-prefix`, `--bindir`, `--datarootdir` and `--mandir`, and most importantly, makes it thus install manpages in the default location for manpages as shown in `./configure --help`, which is $prefix/share/man, not $prefix/man where it installed them before.