* Makefile: add clean target

While working on pict, it's useful to get a clean state.  This clean
target removes the target binary and output generated by a self test
execution (make test).

Signed-off-by: Cleber Rosa <crosa@redhat.com>

* Makefile: allow parallelization of build

The current rule for building pict is based on a single command
execution, which makes faster builds leveraging parallel execution
impossible.

This change introduces the objects that are required to build pict,
and relies on implicit rules to turn the ".cpp" files into the object
files.

After these changes, a command such as "make -j8" shows the benefits
of this change.

Signed-off-by: Cleber Rosa <crosa@redhat.com>

* api/pictapi.h: use default calling convention when using gcc

gcc doesn't understand the __stdcall calling convention.  Let's
use its default when using it as a compiler.

Signed-off-by: Cleber Rosa <crosa@redhat.com>

* Makefile: relax a bit more the compiler options

I reckon that pict should be compiled with all warnings enabled,
and all warnings being treated as errors.  This is actually the goal
of other changes to be proposed soon.

For now, some code, such as naming attributes with the same name as
previously declared types is not really welcomed by some compilers
such as GCC, being considered "nonconformant code".

Since changes to fix this would bring changes to the pict API, let's
keep things simple and simply allow the unchanged code to build on
other compilers such as GCC.

Signed-off-by: Cleber Rosa <crosa@redhat.com>

* Makefile: use default compiler set on system

Users can choose selected compiler by setting the "CXX" environment
variable or by editting the Makefile.

Let's not hardcode a particular compiler, now that both clang and gcc
can be used to compile pict.

Signed-off-by: Cleber Rosa <crosa@redhat.com>

* Makefile: make test depend on target being built

Signed-off-by: Cleber Rosa <crosa@redhat.com>

* Makefile: add source target

It's useful to generate a snapshot of the current state of the GIT
repo.  Let's add a target that generates a tarball using git-archive
using a naming convention that makes it easy to trace to the commit
that generated this snapshot.

This format also happens to be well suitable for using the source
tarball when packaging it in RPM format.

Signed-off-by: Cleber Rosa <crosa@redhat.com>