imlib2: uses dlopen(), not available in static cases

According to the imlib2 people, if you don't support dlopen(), you
have a "crap box":

image.h:16:4: warning: #warning "your crap box doesn't define RTLD_LOCAL !?"

So, let's disable imlib2 when doing static linking.

Fixes:

  http://autobuild.buildroot.net/results/b5e/b5ee859409cd626b769e8a159026e60ec6f29180/



[Peter: fix imlib2 comment as pointed out by Thomas]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>