This relies on the fact that providing multiple --version-script
combines them all, so we effectively create a new symbol version
that has no global symbol, but hides the std:🧵:_M_start_thread
symbols.
This version script trick happens to work with BFD ld, gold, and lld.
The downside is that when providing multiple --version-script's, ld
doesn't want any of them to have no version at all. So for the libraries
that do already have a version script (through SYMBOLS_FILE), we use a
version where there used to be none, using the library name as the
version. Practically speaking, this binds the libraries a little closer
than they used to be, kind of non-flat namespace on OSX (which is the
default there), meaning the dynamic linker will actively want to use
symbols from those libraries instead of a system library that might
happen to have the same symbol name.
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