Long story short, m4/libcurl.m4 exports LIBCURL_CFLAGS but configure.ac imports LIBCURL_CPPFLAGS. If no other component adds -I compiler flag with a path where libcurl headers are installed (by accident) Zabbix compilation will fail.

Not a regression from ZBX-4148 this time

Seems like it's a very old bug, running (in 2.0 branch)

$ svn diff m4/libcurl.m4 configure.in -c 5885

reveals that LIBCURL_CPPFLAGS was renamed to LIBCURL_CFLAGS in m4 script but configure script wasn't synced.