directfb: add an option for debugging support

DirectFB provides two options for debug:
* --enable-debug-support (default is yes)
* --enable-debug (default is no)
The first one provides the functions to print debug information. These
functions can be called inside DirectFB, or by an external DirectFB
driver in another package.
The second one enables verbose mode in DirectFB, using these functions.

Buildroot currently provides an option for the second one only. The
first one - which is necessary for the second one - is always enabled,
because this is the default value.

The new option (BR2_PACKAGE_DIRECTFB_DEBUG_SUPPORT) makes it possible
to disable debugging support (--disable-debug-support) in order to
build the library with no debug feature at all.

When debugging support is disabled, the target installation path of
drivers (/usr/lib/directfb-<version>) is suffixed with -pure. The
public library (/usr/lib/libdirectfb.so) remains unchanged.

Some external DirectFB drivers will install into
/usr/lib/directfb-<version>-pure in release mode. For that reason, it
is important that DirectFB can be configured to use this directory.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>