Add error check in configure

[Tim Josling]

"Joseph S. Myers" wrote:

> there are two
> options for the fix:
> 
> (a) document in sourcebuild.texi the additional places (with any
> appropriate crossreference to gty.texi); or
> 
> (b) changing the interface so that gengtype.c doesn't need to be changed
> for each front end, and documenting the result if front ends still need to
> provide files beyond what's currently documented (e.g., if the mechanism
> used is like lang-options.h and lang-specs.h rather than generating the
> relevant information from the directories present at configure time).
> 
> --
> Joseph S. Myers
> jsm28@cam.ac.uk


OK. I will do (a) soon and (b) later.

Tim Josling