package/libdcadec: enable shared build

package/libdcadec: enable shared build

The libdcadec package currently only builds a static library, which is
naturally built without -fPIC (since this is generally not needed for
static libraries).

However, when the generated static library gets used inside a larger
dynamic library, such as ffmpeg's one, the link fails, since we're
trying to include in a shared library non-PIC code.

To solve this, this commit changes the way the libdcadec library is
built to make sure a shared variant of the library gets built when
appropriate. This way, ffmpeg has a PIC-enabled libdcadec library to
link with.

Fixes
http://autobuild.buildroot.net/results/bd3/bd34d1d2f2d35455d1ddefa1ec247c78f341603f/
http://autobuild.buildroot.net/results/4f7/4f7c87e47e763ce656c50b5862408b18e6f185ab/
http://autobuild.buildroot.net/results/998/99836db091af610a99dbc815b7fdf09cc1d531d1/
http://autobuild.buildroot.net/results/a19/a19023e094cbed491444665d6839a9e65a8eee6c/
http://autobuild.buildroot.net/results/98d/98dde028d1d6199f05c904b498bc39bbaa112aa6/
http://autobuild.buildroot.net/results/4ff/4ffcae6cae419df35e8ca29d429ee178bcf31882/
http://autobuild.buildroot.net/results/8ce/8ced16874255ace4923f6b8888c3fca07f28b804/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>