Problems with gcc + exceptions + -pedantic + libiberty.h

[DJ Delorie]

Since Linux is specifically mentioned in libiberty.h, and linux is the
one having the problem, I suppose it would be acceptable for the
linux-specific case to do something different in libiberty.h.

However, a better solution is to add the gcc-specific prototype
detection logic to some common area, and teach projects to use it.
Then, libiberty.h can default to "no prototype" unless the detection
tells it what to do.