Build HMAC module with -D_BSD_SOURCE -D_DEFAULT_SOURCE per upstream Modules/Setup

Author: geofft

cpython-unix/extension-modules.yml

@@ -326,6 +326,9 @@ _hmac:
     - :libHacl_Hash_SHA2.a
     - :libHacl_Hash_SHA3.a
     - :libHacl_Hash_MD5.a
+  defines:
+    - _BSD_SOURCE
+    - _DEFAULT_SOURCE
 
 _imp:
   config-c-only: true