Silence warnings from linker when run from doctest

```
clang: warning: argument unused during compilation: '-no-pie' [-Wunused-command-line-argument]
tests/haskell_doctest/Bar.hs:11: failure in expression `bar'
expected: 9
 but got: ld: warning: -undefined dynamic_lookup may not work with chained fixups
          ^
          9
Examples: 3  Tried: 2  Errors: 0  Failures: 1
```
If the linker writes some output to stderr, this interferes with the expected output
in the doctest. Note, that if the linker fails, the output will be written to stdout/stderr
in any case.

Author: avdv

haskell/doctest.bzl

@@ -148,6 +148,9 @@ def _haskell_doctest_single(target, ctx):
         {env}
         # doctest needs PATH to call GHC and the C compiler and linker.
         export PATH
+        # signal our cc_wrapper to silence linker outputs as GHC < 9.4 writes that to
+        # the GHCI ouput which interferes with doctest's expected ouput
+        export RULES_HASKELL_SILENCE_LINKER=1
         {doctest} "$@" {inputs} > {output} 2>&1 || (rc=$? && cat {output} && exit $rc)
         """.format(
             doctest = toolchain.doctest[0].path,

haskell/private/cc_wrapper.py.tpl

@@ -570,7 +570,10 @@ def link(output, libraries, rpaths, args):
         rpaths = shorten_rpaths(rpaths, libraries, output)
 
     args.extend(rpath_args(rpaths))
-    run_cc(args, exit_on_error=True)
+    # Note: `RULES_HASKELL_SILENCE_LINKER` is only set if called from doctest,
+    #       which is used to silence the linker output to not interfere with the output
+    #       from GHCi
+    run_cc(args, capture_output="RULES_HASKELL_SILENCE_LINKER" in os.environ, exit_on_error=True)
 
     if is_darwin():
         darwin_rewrite_load_commands(darwin_rewrites, output)