Otherwise we end up with an add without in requirements.

* No \n at end; panic() adds one
* No punctuation mark at end
* Lowercase first letter

This is in preparation for a removal of the generic arch_irn_flag_modify_flags

Didn't notice this before because gcc doesn't report a warning for this,
clang does.

It has been empty for nearly all systems. People who used to put stuff
in config.h can still create a config.h on their own and inject
-include (gcc) or /FI (msvc) into their CPPFLAGS.