m4-not-needed

[Emmanuel Thomé]

Hi,

When gmp is configured with no assembly, as is done e.g. when gcc is
compiled with the gmp source present, the following environment variable
gets set from within ./configure:

    M4=m4-not-needed

Indeed, as there is no assembly code to manipulate with m4, m4 is not
needed.

However, the M4 variable seems to have an unfortunate effect on flex:

ethome at suno-7:/tmp/gcc-build/gmp$ M4=m4-not-needed flex conftest.l 
flex: fatal internal error, exec failed

(where conftest.l is the one from the gmp configure, for that matter).

Under *some* circumstances (I come across this deterministically while
running a gcc compile from a batch job, however I haven't been able to
reproduce it live), it leads to lex.yy.c not being created, eventually
causing the configure script to abort (possibly a sigpipe/wait/unlink
race).

I see that the mercurial code now leaves the variable M4 untouched in
case it already exists, which seems more reasonable than overriding it
with bogus content unconditionally. However I wonder why gmp touches the
M4 variable in the first place in this case. May I suggest leaving this
variable untouched instead ? Or is there a compelling reason for setting
m4-not-needed ?

Best,

E.