Buildsystem: Drop harcoded -shared -fPIC flags

This fix gets around a libtool's "cannot build a shared library"
error when crosscompiling/linking media-engine-simple with
Android's NDK r8e toolchain. These flags are being correctly
handled by auto*/libtool (as they probably should).

https://bugzilla.gnome.org/show_bug.cgi?id=707146