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]

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