Fix incorrect building of libgdiplus on Mavericks bots. Fixes #23553.
There is an #include file resolution problem when C_INCLUDE_PATH is set to the install prefix (as bockbuild does), leading to a build where CAIRO_HAS_QUARTZ_SURFACE is not defined, so GdipCreateFromContext_macosx() wasn't built. I couldn't decipher what exactly changes in the includes when the prefix path loses precedence (when C_INCLUDE_PATH is unset so it's only in CFLAGS), and the Xcode toolchain selected didn't seem to matter, but indeed unsetting C_INCLUDE_PATH results in fixing the build. So this will have to do for now.

C_INCLUDE_PATH should slowly be removed from the global environment anyway, but it seems to change other things too so the global change needs time/auditing.

The problem only showed up on the move to the Mavericks bots and shows up on local builds too.