feat: add c99 mode in build.rs for issue BurntSushi#33

Author: Peefy

pcre2-sys/build.rs

@@ -85,6 +85,8 @@ fn main() {
     {
         builder.debug(true);
     }
+    // Enable c99 mode to compile pcre2 for issue #33
+    builder.flag("-std=c99");
     builder.compile("libpcre2.a");
 }