tb87 failing with "error: undefined reference to 'icu_50::RuleBasedCollator::cloneBinary(unsigned char*, int, UErrorCode&)'"

[Stephan Bergmann]

I don't not what started to cause it, but many 
gerrit_linux_clang_dbgutil builds on tb87 started to fail with

> /home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/i18npool/source/collator/gencoll_rule.cxx:124: error: undefined reference to 'icu_50::RuleBasedCollator::cloneBinary(unsigned char*, int, UErrorCode&)'
> /home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/i18npool/source/collator/gencoll_rule.cxx:128: error: undefined reference to 'icu_50::RuleBasedCollator::cloneBinary(unsigned char*, int, UErrorCode&)'
> /home/tdf/lode/opt_private/gcc-7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:825: error: undefined reference to 'icu_50::UMemory::operator new(unsigned long)'
> /home/tdf/lode/opt_private/gcc-7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:825: error: undefined reference to 'icu_50::UnicodeString::UnicodeString(unsigned short const*)'
> /home/tdf/lode/opt_private/gcc-7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:825: error: undefined reference to 'icu_50::RuleBasedCollator::RuleBasedCollator(icu_50::UnicodeString const&, UErrorCode&)'
> /home/tdf/lode/opt_private/gcc-7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:825: error: undefined reference to 'icu_50::UnicodeString::~UnicodeString()'
> /home/tdf/lode/opt_private/gcc-7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:825: error: undefined reference to 'icu_50::UnicodeString::~UnicodeString()'
> /home/tdf/lode/opt_private/gcc-7.3.0/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:825: error: undefined reference to 'icu_50::UMemory::operator delete(void*)'
> clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)
> /home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/solenv/gbuild/LinkTarget.mk:731: recipe for target '/home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/workdir/LinkTarget/Executable/gencoll_rule' failed
> make[1]: *** [/home/tdf/lode/jenkins/workspace/lo_gerrit/Config/linux_clang_dbgutil_64/workdir/LinkTarget/Executable/gencoll_rule] Error 1

(e.g., <https://ci.libreoffice.org/job/gerrit_linux_clang_dbgutil/95827/>).

I'm not sure whether this is due to some recent commit that does not 
work with certain configurations, or whether it is a genuine 
misconfiguartion of tb87 (I assume the former, though), but I took tb87 
offline for now to avoid further failing builds.

If anybody has a fix for this, please re-enable 
<https://ci.libreoffice.org/computer/tb87/> afterwards.