Do not pass the -fms-extensions flag, it is unsupported on OpenBSD and
breaks the build.

Index: Build.PL
--- Build.PL.orig
+++ Build.PL
@@ -100,7 +100,7 @@ $mb->create_build_script();
 sub _cc_flags {
     my $mb = shift;
 
-    my %unique = map { $_ => 1 } qw( -std=c99 -fms-extensions -Wall -g ),
+    my %unique = map { $_ => 1 } qw( -std=c99 -Wall -g ),
         @{ $mb->extra_compiler_flags || [] },
         _int64_define(),
         _int128_define();