- fix iconv detection for boost.locale on iOS

[SSE4]

• clang-darwin doesn't need --start-group
• clang-darwin doesn't need -lrt

Proposed changes

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see bug reports:
conan-io/conan-center-index#6490
conan-io/conan-center-index#3992
boostorg/build#365
boostorg/build#678
boostorg/build#438

TLDR: building boost for iOS fails with the following error:

clang-darwin.link /Users/spaceim/.conan/data/boost/1.76.0/_/_/build/7ab220ce003d7c6ffe8c029987b8bf98cb7ba452/boost/bin.v2/standalone/ac/clng-drwn-12.0/rls/trgt-os-iphn/thrd-mlt/vsblt-hdn/bz2

    "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++" "-isysroot" "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.2.sdk" "-arch" "arm64" -stdlib=libc++ -L"/Users/spaceim/.conan/data/bzip2/1.0.8/_/_/package/402ac23a56db5265741eb8710725745ae6d028fa/lib" -o "/Users/spaceim/.conan/data/boost/1.76.0/_/_/build/7ab220ce003d7c6ffe8c029987b8bf98cb7ba452/boost/bin.v2/standalone/ac/clng-drwn-12.0/rls/trgt-os-iphn/thrd-mlt/vsblt-hdn/bz2" "/Users/spaceim/.conan/data/boost/1.76.0/_/_/build/7ab220ce003d7c6ffe8c029987b8bf98cb7ba452/boost/bin.v2/standalone/ac/clng-drwn-12.0/rls/trgt-os-iphn/thrd-mlt/vsblt-hdn/main-bz2.o"  -Wl,--start-group -Wl,-Bstatic  -Wl,-Bdynamic -lrt -lbz2 -Wl,--end-group -fPIC -pthread -fvisibility=hidden -fvisibility-inlines-hidden -L/Users/spaceim/.conan/data/libbacktrace/cci.20210118/_/_/package/108e68814533f28957889ee7075ad46a83968698/lib

ld: unknown option: --start-group
clang: error: linker command failed with exit code 1 (use -v to see invocation)

it's due to gcc.jam defining START_GROUP for everything except some while-listed targets. the list includes darwin (desktop macOS), but not iPhone (iOS). PR adds iOS to the white-list.

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• Refactoring (no functional changes, no api changes)
• Documentation content changes
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Further comments

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[Kojoley]

Is this actually a fix? You are disabling threading support for the platform because linker does not support library grouping. Since you use Clang, is not lld available for you?

[SSE4]

Is this actually a fix? You are disabling threading support for the platform because linker does not support library grouping. Since you use Clang, is not lld available for you?

it's a fix. I am not disabling threading, just don't link to librt (which doesn't exist for iOS anyway)
lld doesn't seem to be available:

xcrun -find lld
xcrun: error: sh -c '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -find lld 2> /dev/null' failed with exit code 17664: (null) (errno=Invalid argument)
xcrun: error: unable to find utility "lld", not a developer tool or in PATH

[Kojoley]

I am not disabling threading, just don't link to librt (which doesn't exist for iOS anyway)

You are removing -pthread too.

lld doesn't seem to be available:

xcrun -find lld
xcrun: error: sh -c '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -find lld 2> /dev/null' failed with exit code 17664: (null) (errno=Invalid argument)
xcrun: error: unable to find utility "lld", not a developer tool or in PATH

The executable usually named ld.lld, try to compile and link with clang -fuse-ld=lld, check if gold linker available if there no lld.

[SSE4]

You are removing -pthread too.

it doesn't appear to be needed per https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/pthread.3.html:

The default system libraries include pthread functions.  No additional
     libraries or CFLAGS are necessary to use this API.

The executable usually named ld.lld, try to compile and link with clang -fuse-ld=lld, check if gold linker available if there no lld.

$ clang -fuse-ld=lld /dev/null
clang: error: invalid linker name in argument '-fuse-ld=lld'

$ xcrun -find ld.lld
xcrun: error: sh -c '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -sdk macosx -find ld.lld 2> /dev/null' failed with exit code 17664: (null) (errno=No such file or directory)
xcrun: error: unable to find utility "ld.lld", not a developer tool or in PATH

$ ld.lld
-bash: ld.lld: command not found

anyway, darwin (desktop macOS) is already excluded from the threading list, and I'd expect iphone/appletv to have the same behavior, as they share the same compiler/linker.

[Kojoley]

You are removing -pthread too.

it doesn't appear to be needed per developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/pthread.3.html:

The default system libraries include pthread functions.  No additional
     libraries or CFLAGS are necessary to use this API.

That should have been in the commit message, or better as a comment in the code.