Steps to reproduce:

1. Get system with old libcurl that cannot be upgraded
2. Install new curl in other location (e.g. /usr/local)
3. Build server with ./configure --enable-server --with-libcurl=/usr/local/bin/curl-config

Result:

../../src/libs/zbxhttp/libzbxhttp.a(http.o): In function `zbx_http_convert_to_utf8':
/usr2/alex/home/devel/zabbix/src/libs/zbxhttp/http.c:957: undefined reference to `curl_easy_header'

Expected:

Successful build

Analysis:

Presumably, even though curl-config returns path to libcurl to use for linking, ./configure does not account the proper path so make links against old lib in standard location.