[CMake] correct dependencies of update-if-different generated files?`

[Matthew Woehlke]

Let's say¹ I have a code generation tool, 'gen', that I am naïvely using 
to build a library, like so:

add_custom_command(
   OUTPUT a.cpp b.cpp
   COMMAND gen in.xml
   DEPENDS in.xml
)
add_library(foo a.cpp b.cpp)

Now let's say that 'gen' is clever and does not overwrite the output 
files with identical content (sort-of like configure_file).

Now, if in.xml is changed, the 'gen' command will run every time because 
the mtime of in.xml is newer than that of a.cpp and b.cpp.

BUT... this "only" affects gmake (at least, it does not affect ninja² 
and reportedly does not affect VS).

Has anyone run into this before, and/or is there a way to express the 
build rules so that the build will completely converge (will not execute 
any commands) *without* unnecessarily recompiling the generated source 
files?

And, since I don't believe that there is, how would folks feel about 
improving add_custom_command to specifically handle this case? I think 
in pure make it is possible to solve the problem by having the custom 
command also update a sentinel file on successful completion, and doing 
like:

a.cpp b.cpp: sentinel
   # No command here!

sentinel: in.xml
   gen in.xml
   touch sentinel

Such an enhancement would also be desirable as it would allow 
'automatic' adjusting of the build rules for generators that have the 
problem with only a minor change to the CMake rules, and without 
penalizing generators that are more clever.

(¹ Original/actual code where I ran into this: 
https://codereview.qt-project.org/61497)

(² Ninja is not affected because it records the last time it updated the 
output files independent of their mtime.)

-- 
Matthew