package/minidlna: Fix static compile

Fixes
http://autobuild.buildroot.net/results/aeb/aeb5481526497a01ada8ee9403f282a2374b0383/
http://autobuild.buildroot.net/results/255/25536df514477f3210caf4af27f2f107683f7fb2/
http://autobuild.buildroot.net/results/fa1/fa11cb360190b02d08e74997b0abc73079760dfa/
and many others

This is a follow-up for these patches:
http://patchwork.ozlabs.org/patch/493360/
http://patchwork.ozlabs.org/patch/416007/



[Thomas:
  - don't pass --static to pkg-config, since our pkg-config wrapper
    already passes it automatically when BR2_STATIC_LIBS=y.
  - make only one call to pkg-config for all three libraries
  - put the libintl logic separately from the definition of
    MINIDLNA_CONF_ENV.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>