Previously, this script would create a file the same name as the desired
output, and then move it to a file with .save appended to it if the
check failed. This is problematic with parallel builds because there is
no dependency set up between the actual header file and the one being
created. Thus, it is possible for some compiles to pick up the created
file before it is moved or deleted.
Instead, just create the file with .save appended to it so that no
compile can every accidently pick it up.
The previous behavior of the pyexpander check (in
VTKmCheckPyexpander.cmake) was to generate the file in the binary
directory and then remove it from there iff the check failed. This
caused two problems. The first is that if the check failed then the file
was deleted and there was no file to copy to the source as the
instructions suggested. The second is that if the check succeeded the
build would then use the files in the build directory rather than the
source directory, and if the programmer accidently modified the binary
files (because, for example, if a build error occured there), the
configure system would not catch that.
This change in behavior was that if the check failed, the file is
renamed to have a .save extension so that the file remains and can be
easily copied back to the source directory if that is appropriate. If
the check succeeds, the generated file is removed and a file with the
extension .check is touched. That .check file is used as a make target
to signal that the test has been performed.
Although we cannot expect every developer to have pyexpander, for those
that do the build will automatically run it and check the expanded file
in the source code. If they match, a descriptive error is given.
We don't automatically update the file because subtle problems might
occur. It is better to alert a developer to fix the problem properly.
