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>

package/libvips/libvips.mk

@@ -12,6 +12,16 @@ LIBVIPS_LICENSE = LGPLv2.1+
 LIBVIPS_LICENSE_FILES = COPYING
 # We're patching gtk-doc.make, so need to autoreconf
 LIBVIPS_AUTORECONF = YES
+
+# Sparc64 compile fails, for all optimization levels except -O0. To
+# fix the problem, use -O0 with no optimization instead. Bug reported
+# upstream at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69038.
+ifeq ($(BR2_sparc64),y)
+LIBVIPS_CXXFLAGS += -O0
+endif
+
+LIBVIPS_CONF_ENV += CXXFLAGS="$(TARGET_CXXFLAGS) $(LIBVIPS_CXXFLAGS)"
+
 LIBVIPS_CONF_OPTS = \
 	--disable-introspection \
 	--without-dmalloc \