When building from a distinct source directory, the build directory is naturally
empty, so make tries to build graminit.c and graminit.h and move them to their
natural location - the source directory.  When the source directory is a read-only
filesystem that kills the build, and anyway it's inappropriate.  There may be a
better way to express the rule, but previous releases seem to have mostly omitted
it anyway.

I've checked in, IMHO, a better solution.  The grammar
source files are now generated in the build directory.  If
this doesn't work for a read-only source dir please let me
know.

D'oh, your right.  Can you look at patch #103930?  I don't
want to omit the rule.  Prevous versions of Python did
omit it but that caused some surprises when people started
hacking on the grammar.  

Actually this is only a more complicated way to do the
same thing - now you generate the files, but you still
don't use them.  Not graminit.h, anyway.  Here's what
happens if I remove graminit.h from the source directory:

cc -c -g -O2 -Wall -Wstrict-prototypes -I.
  -I/tulsa/src/python-2.1a2b/Include -DHAVE_CONFIG_H 
  -o Python/compile.o /tulsa/src/python-2.1a2b/Python/compile.c
/tulsa/src/python-2.1a2b/Python/compile.c:17: graminit.h: No such file or directory

You would have to add -IInclude to CFLAGS, along with
-I$(srcdir)/Include;  otherwise --srcdir build with
modified grammar is broken (whether srcdir is read only
or read/write.)