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UseSWIG: remove duplicate directory includes

Seth R Johnson requested to merge sethrj/cmake:useswig-includes into master

This changeset simply removes duplicate -I entries from target-propagated includes (with the USE_TARGET_INCLUDE_DIRECTORIES source file property) in the SWIG command.

The motivation is that our project (SCALE) recently encountered a build failure while trying to generate SWIG wrappers for a library that was downstream of almost everything in our codebase:

execvp: /bin/sh: Argument list too long

The problem is that the TriBITS build system links each library publicly against all N upstream dependencies, each of which is linked against up to N-1 upstream deps. It seems that $<TARGET_PROPERTY:${target_name},INCLUDE_DIRECTORIES> does not automatically deduplicate include transitive directory includes, so the SWIG link line can contain at worst O(N^2) include commands pointing to N include dirs.

It may be worth examining whether the TARGET_PROPERTY command itself should automatically do the deduplication. Until then, this provides an easy fix.

See https://gitlab.kitware.com/sethrj/swig-include-dirs for a demonstration of the difference. With 32 libraries, each of which links against all parents:

Words in COMMAND before fix:
     553
Words in COMMAND after fix:
      58
Edited by Seth R Johnson

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