TEMP_FAILURE_RETRY is glibc specific

Trying to build release 0.2.0 against musl c-library:
```
CC bwrap-bubblewrap.o
./bubblewrap.c: In function 'main':
./bubblewrap.c:2158:18: error: implicit declaration of function 'TEMP_FAILURE_RETRY' [-Werror=implicit-function-declaration]
(void) TEMP_FAILURE_RETRY (read (opt_userns_block_fd, b, 1));
^~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
```

The above is glibc specific. Perhaps you could `#define` it in code if not defined, or just add own macro to do the same. This would improve portability for non-glibc systems such as musl c-library.