Use correct parameter types when calling curl_easy_setopt()

[mkauf]

Fix compiler warnings, including this one:

In file included from /home/test/curl/include/curl/curl.h:2515:0,
                 from geoipupdate.h:6,
                 from geoipupdate.c:2:
geoipupdate.c: In function 'download_to_file':
/home/test/curl/include/curl/typecheck-gcc.h:56:9: warning: call to '_curl_easy_setopt_err_write_callback' declared with attribute warning: curl_easy_setopt expects a curl_write_callback argument for this option
         _curl_easy_setopt_err_write_callback();                               \
         ^
geoipupdate.c:538:5: note: in expansion of macro 'curl_easy_setopt'
     curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, get_expected_file_md5);
     ^

[horgh]

Hi @mkauf. Thanks very much for the contribution!

The changes look good.

I am curious about how you saw these warnings. I'd like to ensure we see any similar ones going forward! Are you able to tell me anything about the OS/compiler version/curl version you're using, and any flags you used to enable them?

[mkauf]

I have used a recent curl version (7.54.0). Probably old curl versions do not trigger this warning.

You also need to compile with GCC 4.3 or later to get this warning, see https://github.com/curl/curl/blob/master/include/curl/curl.h :

#if defined(__GNUC__) && defined(__GNUC_MINOR__) && \
    ((__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)) && \
    !defined(__cplusplus) && !defined(CURL_DISABLE_TYPECHECK)
#include "typecheck-gcc.h"
#else
...

[horgh]

Interesting! Thanks for describing that.

I found that I needed to pass -Wsystem-headers to gcc in order to see the warning in my environment.

As far as being able to see such warnings in the future, I don't think adding -Wsystem-headers to the default CFLAGS would be appropriate for us. I tried adding the flag for our CI builds, but it unfortunately does not raise this warning. I'm wondering if it is due to different gcc/libcurl versions.

Anyway, your change looks great! Thank you again!