configure: memalign is broken on djgpp 2.05

djgpp 2.05 finally provides posix-compatible memalign, but it is broken,
so use malloc instead.

discussion: http://www.delorie.com/archives/browse.cgi?p=djgpp/2017/12/29/16:26:58

configure

@@ -6796,10 +6796,17 @@ check_deps $CONFIG_LIST       \
 enabled threads && ! enabled pthreads && ! enabled atomics_native && die "non pthread threading without atomics not supported, try adding --enable-pthreads or --cpu=i486 or higher if you are on x86"
 enabled avresample && warn "Building with deprecated library libavresample"
 
-if test $target_os = "haiku"; then
+case $target_os in
+haiku)
     disable memalign
     disable posix_memalign
-fi
+    ;;
+*-dos|freedos|opendos)
+    if test_cpp_condition sys/version.h "defined(__DJGPP__) && __DJGPP__ == 2 && __DJGPP_MINOR__ == 5"; then
+        disable memalign
+    fi
+    ;;
+esac
 
 flatten_extralibs(){
     nested_entries=