Declare Z_HAVE_UNISTD_H on non-windows platforms for zlib

This avoids an error when compiling zlib with ISO C99 conforming compilers:
```
external/zlib.hs/gzwrite.c:78:20: error: call to undeclared function 'write'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
            writ = write(state->fd, strm->next_in, put);
                   ^
external/zlib.hs/gzwrite.c:78:20: note: did you mean 'fwrite'?
/nix/store/51c1k4sj0irwgyvkgbyhqi0ggilyh3j2-Libsystem-1238.60.2/include/stdio.h:254:9: note: 'fwrite' declared here
size_t   fwrite(const void * __restrict __ptr, size_t __size, size_t __nitems, FILE * __restrict __stream) __DARWIN_ALIAS(fwrite);
         ^
external/zlib.hs/gzwrite.c:108:24: error: call to undeclared function 'write'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                writ = write(state->fd, state->x.next, put);
                       ^
external/zlib.hs/gzwrite.c:627:9: error: call to undeclared function 'close'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    if (close(state->fd) == -1)
        ^
3 errors generated.
```

Author: avdv

examples/non_module_deps.bzl

@@ -21,7 +21,16 @@ cc_library(
     includes = ["."],
     visibility = ["//visibility:public"],
 )
-cc_library(name = "z", srcs = glob(["*.c"]), hdrs = glob(["*.h"]))
+cc_library(
+    name = "z",
+    srcs = glob(["*.c"]),
+    hdrs = glob(["*.h"]),
+    copts = select({
+        "@bazel_tools//src/conditions:windows": [],
+        # Needed to avoid "call to undeclared function" errors [-Wimplicit-function-declaration]
+        "//conditions:default": ["-DZ_HAVE_UNISTD_H"],
+    }),
+)
 """,
         sha256 = "b5b06d60ce49c8ba700e0ba517fa07de80b5d4628a037f4be8ad16955be7a7c0",
         strip_prefix = "zlib-1.3",

rules_haskell_tests/non_module_deps.bzl

@@ -89,9 +89,11 @@ cc_library(
     name = "z",
     srcs = glob(["*.c"]),
     hdrs = glob(["*.h"]),
-    # Needed because XCode 12.0 Clang errors by default.
-    # See https://developer.apple.com/documentation/xcode-release-notes/xcode-12-release-notes.
-    copts = ["-Wno-error=implicit-function-declaration"],
+    copts = select({
+        "@bazel_tools//src/conditions:windows": [],
+        # Needed to avoid "call to undeclared function" errors [-Wimplicit-function-declaration]
+        "//conditions:default": ["-DZ_HAVE_UNISTD_H"],
+    }),
     # Cabal packages depending on dynamic C libraries fail on MacOS
     # due to `-rpath` flags being forwarded indiscriminately.
     # See https://github.com/tweag/rules_haskell/issues/1317