Generate porting.info in build tree rather than source tree

Hello,

With the recent improvements in the build system by Mike Frysinger, I've 
noticed a minor regression.

When building newlib, the file libgloss/doc/porting.info is always 
created in the source tree rather than the build tree. Looking at the 
libgloss/Makefile.* files, it appears that this is derived from the 
libgloss/doc/Makefile.inc line:
info_TEXINFOS += %D%/porting.texi

To my knowledge (no automake wiz...), %D% is a relative path, so I 
suppose the generated path should not use $(srcdir), yet the Makefile.in 
does contain $(srcdir)/doc/porting.info.

Would it be possible to have this porting.info file generated in the 
build tree rather than the source tree to avoid having the source tree 
in a dirty state after a build?

I've spotted this building newlib for arm-none-eabi.

Kind regards,
Torbjörn