Too many hangs when a large header set is included (e.g. webkitgtk)

[Ambrevar]

If I include a large header set like webkitgtk:

(setq c-eldoc-includes "`pkg-config gtk+-3.0  --cflags` -I./ -I../ `pkg-config webkit2gtk-4.0 --cflags`")

then every out-of-cache lookups hangs for some seconds. This makes this package unusable. I haven't profiled it, but there might be some way to optimize this.

[nflath]

Yeah, I suspect this is going to be due to running the preprocessor. Fixing this would be tons of work.