osutil: allow disabling setprocname via a define passed to the compiler

In some situations, we run a custom python launcher that appears to not set up
Py_GetArgcArgv correctly. We then proceed to promptly crash when we attempt to
dereference NULL. Being able to completely disable setprocname is beneficial in
these situations, since we won't even attempt to use it, even if the case that
causes the crash is fixed.

Right now, if I compile osutil.so with -DSETPROCNAME_USE_NONE, the compilation
fails on python3 due to SETPROCNAME_USE_NONE redefinition. I could possibly
work around that, but it's likely helpful to have a way of disabling this
completely without it being brittle (i.e. if python3 ever gains the ability to
perform this operation).

Differential Revision: https://phab.mercurial-scm.org/D6865

mercurial/cext/osutil.c

@@ -765,6 +765,8 b' bail:'
 
 #endif /* CMSG_LEN */
 
+/* allow disabling setprocname via compiler flags */
+#ifndef SETPROCNAME_USE_NONE
 #if defined(HAVE_SETPROCTITLE)
 /* setproctitle is the first choice - available in FreeBSD */
 #define SETPROCNAME_USE_SETPROCTITLE
@@ -775,6 +777,7 b' bail:'
 #else
 #define SETPROCNAME_USE_NONE
 #endif
+#endif /* ndef SETPROCNAME_USE_NONE */
 
 #ifndef SETPROCNAME_USE_NONE
 static PyObject *setprocname(PyObject *self, PyObject *args)