parallel make fails for guile 1.8

[Mattias Holm]

Makes fails  when parallelisation option -jN is used. This is at  
least true for Mac OS X (which also fail in other parts of the make  
process, though this later error are apparently known (this also  
prevents me from using Guile 1.8 in my project)).

Non the less, something in the line of this results from parallel makes:

./guile-snarf -o arbiters.x arbiters.c -DHAVE_CONFIG_H -I.. -I.. -I/ 
opt/gmp/include -g -O2 -Wall -Wmissing-prototypes -Werror
In file included from ../libguile/_scm.h:63,
                 from alist.c:20:
../libguile/__scm.h:76:32: error: libguile/scmconfig.h: No such file  
or directory
In file included from ../libguile/_scm.h:63,