toolchain/48914: Builds randomly fail with "bconfig.h: no such file or directory"

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         48914
>Category:       toolchain
>Synopsis:       Builds randomly fail with "bconfig.h: no such file or 
>directory"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 16 12:00:00 +0000 2014
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:

>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

The automated builds on babylon5 sometimes fail with error messages
like the following:

    
/tmp/bracket/build/2014.06.15.20.14.41-i386/src/tools/gcc/../../external/gpl3/gcc/dist/gcc/gengtype.c:21:21:
 fatal error: bconfig.h: No such file or directory<command-line>:0:0: warning: 
"TARGET_SYSTEM_ROOT" redefined
    <command-line>:0:0: note: this is the location of the previous definition
    
/tmp/bracket/build/2014.06.15.20.14.41-i386/src/tools/gcc/../../external/gpl3/gcc/dist/gcc/gengtype-state.c:27:21:
 fatal error: bconfig.h: No such file or directory

This is affecting all three ports being built on babylon5.  Here is a
table summarizing the number of builds that have failed with the above
error so far, and the CVS date of the earliest source that failed to
build:

  Port     Number of failed builds    CVS date of first failure
  ----     -----------------------    -------------------------
  i386                          14          2014.04.07.14.40.17
  amd64                         10          2014.04.03.18.27.37
  sparc                         12          2014.03.07.05.50.42

The first failed sparc build was the day after sparc was switched
to gcc 4.8, and the first failed i386 build was two days after i386
was switched to gcc 4.8.  This could be a coincidence, but on the
other hand, it might not be.

>How-To-Repeat:

Build NetBSD repeatedly.  Watch it sometimes fail.

>Fix: