Installation failure on Linux

# Error
```
1 | (rule
2 | (targets cflags.sexp)
3 | (deps
4 | (:first_dep cflags.sh))
5 | (action
6 | (bash "./%{first_dep} > %{targets}")))
./cflags.sh: 2: set: Illegal option -o pipefail
```

# [Source](https://github.com/janestreet/bin_prot/blob/master/xen/cflags.sh#L1)
```
#!/bin/sh
set -e -o pipefail
```

# Cause:
`set -e -o pipefail` is a bash construct

# Fix:
```
#!/bin/bash
```

Start with xen/cflags.sh at line 1 and reproduce the installation failure on Linux. Check that the script runs without the reported illegal-option error, then rerun the affected installation or build step to confirm cflags.sexp is generated successfully.

Written by the indexing model from the issue text.