Use a CMake-specific path to this configured header so that it works
when included from a CMake source file in addition to curl sources.
We do not install the Curl headers with CMake, so this hack is okay.
Re-apply change from commit v2.8.0~1683 (add initial support for HAIKU
OS, 2008-09-15) on updated upstream curl. However, leave out the part
that was reverted by commit v3.0.0-rc1~541^2~1 (Haiku: Remove outdated
preprocessor checks, 2013-10-05).
Brad King