Sort files in static library to make the build reproducible

[Bas Couwenberg]

As part of the effort to support Reproducible Builds Alexis Bienvenüe submitted two patches in ​Debian Bug #824808 to sort the object files in the static library: sort-files-1 & sort-files-2

These patches resolve the random_order_in_static_libraries issue which is illustrated by the diffoscope output.

The attached patch is modified version of the sort-files-2 patch doing do wildcard sorting in the $(LIBGDAL) target instead of a separate $(LIBGDAL).buildit target. Alexis chose that approach because:

In some situations some of the target dependencies do not exist before the "make target" call. When this arises, as $(wildcard) is expanded before building the dependencies, it can miss some files… I was not sure of the situation here, so I added a transitional target to be safe.

I prefer the single line change in the attached patch, but you may want to consider the separate target as done in the sort-files-2 patch too.

[Bas Couwenberg]

Adding a description causes an internal server error.

[Bas Couwenberg]

Links to the patches from the Debian bugreport:

• ​sort-files-1
• ​sort-files-2

[Bas Couwenberg]

And the ​reproducible builds issue and diffoscope output:

• ​https://tests.reproducible-builds.org/issues/unstable/random_order_in_static_libraries_issue.html
• ​https://tests.reproducible-builds.org/dbd/unstable/amd64/gdal_2.1.0+dfsg-2.diffoscope.html

[Even Rouault]

In 34257:

GNUmakefile: sort files in static library to make the build reproducible (patch by sebastic, derived from patch by Alexis Bienvenüe, ​https://bugs.debian.org/824808, closes #6520)

[Even Rouault]

Being a fix for a not critical issue, I'm not too keen on applying that to the stable branch.