DecisionTree::dot(name, ...): skip system() shellout on iOS

[ShuangLiu1992]

Problem

Building gtsam for iOS (apple-clang, iphoneos sysroot) fails on every TU that instantiates DecisionTree with:

DecisionTree-inl.h:1081:9: error: 'system' is unavailable: not available on iOS
    system(("dot -Tpdf " + name + ".dot -o " + name + ".pdf >& /dev/null")
    ^

iOS marks std::system() as __attribute__((unavailable)) in <stdlib.h>, so even a never-executed call site fails the compile.

Fix

The PDF rendering inside DecisionTree<L, Y>::dot(const std::string& name, ...) is a debug convenience layered on top of the .dot file the same function always writes. Keep the .dot emission unconditional, gate just the system(...) shellout behind !TARGET_OS_IPHONE. Callers who want a PDF on an iOS-targeted build can run dot -Tpdf <name>.dot -o <name>.pdf themselves on any host that has a shell.

    std::ofstream os((name + ".dot").c_str());
    dot(os, labelFormatter, valueFormatter, showZero);
#if defined(__APPLE__) && TARGET_OS_IPHONE
    // iOS marks std::system() unavailable, breaking the build. The PDF
    // rendering is a debug convenience over the always-emitted .dot file;
    // callers who want a PDF on iOS-targeted builds can run
    // `dot -Tpdf <name>.dot -o <name>.pdf` themselves on a host with a shell.
#else
    int result =
        system(...);
    if (result == -1)
      throw std::runtime_error("DecisionTree::dot system call failed");
#endif

<TargetConditionals.h> is added inside an #if defined(__APPLE__) guard at the top of the file so non-Apple builds don't get a missing header.

Behavior change

• iOS builds: no longer crash to compile. Calling DecisionTree::dot(name, ...) writes the .dot file and skips the PDF rendering.
• Every other platform (macOS, Linux, Windows, etc.): unchanged. TARGET_OS_IPHONE is 0 on macOS and undefined elsewhere; the else branch always selects, and the patch reduces to a no-op preprocessor pass.

Test

Verified by rebuilding gtsam against the iphoneos26.0 SDK with apple-clang: previously the build hit ~24 error: 'system' is unavailable across discrete/Algebraic*, Discrete*, etc.; with this patch the build completes cleanly.

[ProfFan]

Thank you!

[dellaert]

Cool! merging.