🚀 go-pugleaf

 >The Sun make keeps track of the command used for each target, in a
 >.make.state file.  This will do just what you want, of course it's no
 >good if you don't have a Sun.

(Actually, I've seen ads for what I remember as being a PC version of
"make" that claims to be compatible with the Sun version; whether this
includes the ".KEEP_STATE" target and corresponding file I don't know.)

 >But it's so useful that for this one reason I haven't switched to Gnu
 >make.  I think it would be a very useful addition to Gnu make.

I certainly like it as well, although it doesn't do exactly what the
person wanted, i.e. keep track of which files actually use the #define
in question and only rebuild those if the #define changes.

Note, though, that it depends on changes to the compiler (preprocessor
phase) and linker as well, so that whenever a file is #included or a
library is linked in, it's recorded as being a file that the target
depends on.  You can't *just* change GNU Make, you'd have to change GCC
as well.  Sun "make" and "cpp"/"ld" use the environment variable
SUNPRO_DEPENDENCIES to communicate; a sample run yielded the following
environment variables (having a rule that ran "env"):

	KEEP_STATE=
	SUNPRO_DEPENDENCIES=/tmp/make.dependency.20693.0 all

I don't know what KEEP_STATE is supposed to be; SUNPRO_DEPENDENCIES has
what I believe is the pathname of a file into which the program is to
dump the dependencies in question (presumably in Makefile form), but I
don't know what the second token (the "all") does.