package/giflib: fix host build without convert

Do not build docs for host-giflib to avoid the following build failure
without convert on host raised since bump to version 5.2.2 in commit
f98239dada59270efe19de9d9fae50ec9ab9d28c and
https://sourceforge.net/p/giflib/code/ci/d565f6fa04be8973425bd7cd3f169908ac9e95c2:

convert ../pic/gifgrid.gif -resize 50x50 giflib-logo.gif
make[2]: convert: No such file or directory

Fixes: f98239dada59270efe19de9d9fae50ec9ab9d28c
 - http://autobuild.buildroot.org/results/04dd54f5060881cb8aa030de34edb4ceea863fa6

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/giflib/giflib.mk

@@ -27,7 +27,7 @@ define GIFLIB_BUILD_CMDS
 endef
 
 define HOST_GIFLIB_BUILD_CMDS
-	$(HOST_CONFIGURE_OPTS) $(MAKE) -C $(@D)
+	$(HOST_CONFIGURE_OPTS) $(MAKE) -C $(@D) shared-lib
 endef
 
 define GIFLIB_INSTALL_STAGING_CMDS
@@ -42,7 +42,7 @@ endef
 
 define HOST_GIFLIB_INSTALL_CMDS
 	$(HOST_CONFIGURE_OPTS) $(MAKE) -C $(@D) DESTDIR=$(HOST_DIR) \
-		PREFIX=/usr install
+		PREFIX=/usr install-include install-shared-lib
 endef
 
 $(eval $(generic-package))