compile: add the current git commit hash

This is to help identify the output script. Using
commit hash as version is not a good way, though
compile: use VERSION environment to get version information

In case VERSION is empty (or not set), the git commit hash will be used
compile: using short version of commit hash
Makefile: Add :{pacapt,install}.dev sections

The .dev sections are used to generate and install the unstable
(development) version of pacapt. The primary purpose is to
have a stable version pacapt in the same directory
Makefile: clean up output message
New option -V to print pacapt version information

The version information is provided at the compile time,
and is saved into the PACAPT_VERSION variable. A new fucntion
(_print_pacapt_version) is to print this variable and license
information.
pacapt: (re)add the script to help users

Users do not need to compile the script. The pacapt
script (stable version) is built for them. Developers
will use :pacapt.dev instead.
CONTRIBUTING: note about development and stable scripts