Don't use std::thread on Linux

[akx]

This augments #1024 so the use of std::thread on Linux is an option.

This is required to make the build not require GLIBCXX_3.4.29 or newer, which may not be available on older Linux distributions.

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[akx]

Without std::thread the required GLIBCXX version is 3.4.22 (which is fine on e.g. Ubuntu 20.04).

# diff master.txt 1113.txt
1,5c1,5
< /bitsandbytes/libbitsandbytes_cpu.so: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /bitsandbytes/libbitsandbytes_cpu.so)
< 	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fffffdd7000)
< 	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fffffc86000)
< 	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fffffc6b000)
< 	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fffffa79000)
---
> ./libbitsandbytes_cpu.so: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ./libbitsandbytes_cpu.so)
> 	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fffffde0000)
> 	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fffffc8f000)
> 	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fffffc74000)
> 	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fffffa82000)
9c9
< 	/bitsandbytes/libbitsandbytes_cpu.so:
---
> 	./libbitsandbytes_cpu.so:
12a13
> 		libc.so.6 (GLIBC_2.34) => not found
14d14
< 		libstdc++.so.6 (GLIBCXX_3.4.11) => /lib/x86_64-linux-gnu/libstdc++.so.6
16,17d15
< 		libstdc++.so.6 (CXXABI_1.3.9) => /lib/x86_64-linux-gnu/libstdc++.so.6
< 		libstdc++.so.6 (GLIBCXX_3.4.29) => not found
20d17
< 		libstdc++.so.6 (GLIBCXX_3.4.22) => /lib/x86_64-linux-gnu/libstdc++.so.6

[akx]

cc @rickardp as the author of #1024 – does this look sensible?

[akx]

EDIT: okay, now that I checked the built .sos, they still target a too-new GLIBCXX somehow, meh.