RFR: Add Makefile wrapper

[Erik Helin]

Hi all,

this patch adds a Makefile "wrapper" around `gradlew`. This allows the Skara CLI tooling to be installed using the (for Linux/macOS users) familiar commands:

$ make
$ make install

The user can also give a prefix to `make install`, as in `make prefix=/usr install`. The Makefile variables have the same names as for git upstream. One benefit of installing the Skara CLI tooling this way is that the man pages are available via `--help` to various Skara CLI commands.

Please note that this is in _no_ way meant to replace the existing Gradle based build system, this is only a thin wrapper for convenience.

Thanks,
Erik

----------------

Commits:
 - c34d755a: Add Makefile

Changes: https://git.openjdk.java.net/skara/pull/172/files
 Webrev: https://webrevs.openjdk.java.net/skara/172/webrev.00
  Stats: 56 lines in 1 file changed: 56 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/172.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/172/head:pull/172

PR: https://git.openjdk.java.net/skara/pull/172