[R-SIG-Mac] Incremental builds with macOS 27.2 may be broken

[Prof Brian Ripley]

If you want to update a build of R-devel or R-patched after svn up. 
first look near the bottom of file Makeconf.  If the .c.d rule is NOT

.c.d:
         @echo "making $@ from $<"
         @$(CC) -MM $(ALL_CPPFLAGS) $< > $@

then you need to do a clean build, starting with make distclean and 
configure.

The clang in the 27.2 CLT/Xcode distribution has changed the format of 
the output of clang -MM (and -M) and it no longer is what configure used 
to expect, so it does not find a working .c.d rule, and all the 
Makefiles need to be remade to add missing dependencies.

We know that this was changed after CLT 26.6: we think after CLT 27 
(there was no 27.1).

-- 
Brian D. Ripley,                  ripleybd using icloud.com
Emeritus Professor of Applied Statistics, University of Oxford