When asking the mingw-w64 variant of GCC to compile C11 code, it seems
to link implicitly to libwinpthread, which does implement a pthread
emulation (that is more complete than Git's).
In preparation for vendoring in mimalloc (which requires C11 support),
let's keep preferring Git's own pthread emulation.
To avoid linker errors where it thinks that the `pthread_self` and the
`pthread_create` symbols are defined twice, let's give our version a
`win32_` prefix, just like we already do for `pthread_join()`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
c4e5bb652c