Bison 2.7.90 problems...

[Faber, Byron T]

I've been running various snapshots of the bison GIT repository since roughly 
January without any trouble.

My environment:  Cygwin compiled bison, generating code compiled with Visual 
Studio 2010.

With Bison 2.7.90, I'm seeing the following output now, that never happened 
previously:

/usr/bin/m4: m4_syscmd subprocess failed
support/share/bison/c++.m4:33: error: b4_cat: cannot write to stdout
support/share/bison/c++.m4:33: the top level

(I'm using a batch file that points the M4 files at a local folder)

I was successfully using bison:
2.7.1129-2dc8

I can't spot the problem, not being heavily familiar with m4, but it seems to 
be related to something that's brought in with line 705 in bison.m4...

Anybody have suggestions or pointers?
Byron