I tried (with success) this configuration yesterday when building for the first time bfgminer with mingw. I wrote the @CPPFLAG_JANSSON_STATICLIB@ keyword observing the libcurl.pc obtained from http://curl.haxx.se/gknw.net/7.28.1/dist-w32/curl-7.28.1-devel-mingw32.zip that is as follows:

prefix=.
exec_prefix=${prefix}/bin
libdir=${prefix}/lib
includedir=${prefix}/include
supported_protocols="dict file ftp ftps gopher http https imap imaps ldap pop3 pop3s rtmp rtsp scp sftp smtp smtps telnet tftp"
supported_features="AsynchDNS GSS-Negotiate IDN Largefile NTLM SPNEGO SSL SSPI libz"

Name: libcurl
URL: http://curl.haxx.se/
Description: Library to transfer files with ftp, http, etc.
Version: 7.28.1
Libs: -L${libdir} -lcurl -lcurldll
Libs.private:
Cflags: -I${includedir} @CPPFLAG_CURL_STATICLIB@