made install command mac-compatible

[j-krose]

When running make install on MacOS I was get the following output:

% make install
which ronn && make update_docs || true
install -D build/focus-stack "/usr/local/bin/focus-stack"
usage: install [-bCcpSsUv] [-f flags] [-g group] [-m mode] [-o owner]
               [-M log] [-D dest] [-h hash] [-T tags]
               [-B suffix] [-l linkflags] [-N dbdir]
               file1 file2
       install [-bCcpSsUv] [-f flags] [-g group] [-m mode] [-o owner]
               [-M log] [-D dest] [-h hash] [-T tags]
               [-B suffix] [-l linkflags] [-N dbdir]
               file1 ... fileN directory
       install -dU [-vU] [-g group] [-m mode] [-N dbdir] [-o owner]
               [-M log] [-D dest] [-h hash] [-T tags]
               directory ...
make: *** [install] Error 64

man install (on MacOS) says:

     -D destdir
             Specify the DESTDIR (top of the file hierarchy) that the items are installed in to.  If -M
             metalog is in use, a leading string of “destdir” will be removed from the file names logged
             to the metalog.  This option does not affect where the actual files are installed.

which is different than the functionality on linux:

-D
    create all leading components of DEST except the last, or all components of --target-directory, then copy SOURCE to DEST

To avoid this incompatibility I just replaced the -D with a mkdir -p to emulate the same behavior.

After this change, running make install works properly; focus-stack is on my path and man focus-stack shows the proper man page.

[j-krose]

Addresses #63

[j-krose]

@PetteriAimonen FYI

[PetteriAimonen]

Thanks!