libvips: fix compile error for sparc64

Fixes following build error:
http://autobuild.buildroot.net/results/1515f070c0fd410c681fc8306700244c34eb1635/



[Thomas:
 - simplify logic.
 - add reference to submitted gcc bug.]

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>