package/mpv: handle --{en,dis}able-libmpv-{shared,static}

package/mpv: handle --{en,dis}able-libmpv-{shared,static}

libmpv-static and libmpv-shared are disabled by default resulting in the
following build failure when building with gl but without rpi, wayland
or x11:

Checking for OpenGL without platform-specific code (e.g. for libmpv)      : libmpv-shared not found
Checking for OpenGL context support                                       : gl-cocoa not found
You manually enabled the feature 'gl', but the autodetection check failed.

Here is an extract of wscript:

    } , {
        'name': '--plain-gl',
        'desc': 'OpenGL without platform-specific code (e.g. for libmpv)',
        'deps': 'libmpv-shared || libmpv-static',
        'func': check_true,
    }, {
        'name': '--gl',
        'desc': 'OpenGL context support',
        'deps': 'gl-cocoa || gl-x11 || egl-x11 || egl-drm || '
                 + 'gl-win32 || gl-wayland || rpi || '
                 + 'plain-gl',
        'func': check_true,
        'req': True,
        'fmsg': "No OpenGL video output found or enabled. " +
                "Aborting. If you really mean to compile without OpenGL " +
                "video outputs use --disable-gl.",
    }, {

Enabling both the shared and static libraries is not allowed by mpv, so
we consider the BR2_STATIC_LIBS to be static, and otherwise (i.e.
BR2_SHARED_LIBS and BR2_SHARED_STATIC_LIBS) to be shared.

Fixes:
 - http://autobuild.buildroot.org/results/590d2a8b6746ef071dfb439e42b636f81dbdc35d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - expand config log about shared/static icompatibility
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>