Raspbian build fails due to need for libatomic

I'm building CapnP 0.9.1 from source on a new Raspberry Pi 4 running Raspbian (because apt only has version 0.7.) 



```

$ curl -O https://capnproto.org/capnproto-c++-0.9.1.tar.gz

$ tar zxf capnproto-c++-0.9.1.tar.gz

$ cd capnproto-c++-0.9.1

$ ./configure

$ make -j6 check

$ sudo make install

```



Everything went fine until the final "sudo make install" step:



```

...

libtool: link: g++ -I./src -I./src -DKJ_HEADER_WARNINGS -DCAPNP_HEADER_WARNINGS -DCAPNP_INCLUDE_DIR=\"/usr/local/include\" -pthread -O2 -DNDEBUG -pthread -DKJ_HAS_ZLIB -pthread -o .libs/capnp src/capnp/compiler/module-loader.o src/capnp/compiler/capnp.o  ./.libs/libcapnpc.so ./.libs/libcapnp-json.so ./.libs/libcapnp.so ./.libs/libkj.so -lpthread -pthread

/usr/bin/ld: ./.libs/libcapnp.so: undefined reference to `__atomic_load_8'

/usr/bin/ld: ./.libs/libcapnp.so: undefined reference to `__atomic_store_8’

```



After some googling I found that the fix is to link with libatomic -- I simply changed `LIBS` in the Makefile (`LIBS =  -lpthread -latomic`), but I'm aware the Makefile is generated by autoconf; I'm not sure where the list of libraries actually comes from.

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