AIX: add support for shared builds

Signed-off-by: Diego Biurrun <diego@biurrun.de>

configure

@@ -3050,7 +3050,10 @@ enabled spic && enable_weak pic
 # OS specific
 case $target_os in
     aix)
+        SHFLAGS=-shared
         add_cppflags '-I\$(SRC_PATH)/compat/aix'
+        enabled shared && add_ldflags -Wl,-brtl
+        ar_default='ar -Xany'
         ;;
     haiku)
         prefix_default="/boot/common"