{bintools-wrapper,cc-wrapper}: allow paths relative to the Darwin SDK

`-L` and `-I` are interpreted relative to the `$SDKROOT` by the
Darwin toolchain, so we have to avoid filtering out such paths in the
purity filter hacks in order to not break e.g. the .NET Core build
system. It’s also just the correct thing to do for the platform.

pkgs/build-support/bintools-wrapper/ld-wrapper.sh

@@ -44,9 +44,9 @@ if [[ "${NIX_ENFORCE_PURITY:-}" = 1 && -n "${NIX_STORE:-}"
     while (( "$n" < "$nParams" )); do
         p=${params[n]}
         p2=${params[n+1]:-} # handle `p` being last one
-        if [ "${p:0:3}" = -L/ ] && badPath "${p:2}"; then
+        if [ "${p:0:3}" = -L/ ] && badPathWithDarwinSdk "${p:2}"; then
             skip "${p:2}"
-        elif [ "$p" = -L ] && badPath "$p2"; then
+        elif [ "$p" = -L ] && badPathWithDarwinSdk "$p2"; then
             n+=1; skip "$p2"
         elif [ "$p" = -rpath ] && badPath "$p2"; then
             n+=1; skip "$p2"

pkgs/build-support/cc-wrapper/cc-wrapper.sh

@@ -103,7 +103,7 @@ if [[ "${NIX_ENFORCE_PURITY:-}" = 1 && -n "$NIX_STORE" ]]; then
             -[IL] | -isystem) path=$p2 skipNext=true ;;
         esac
 
-        if [[ -n $path ]] && badPath "$path"; then
+        if [[ -n $path ]] && badPathWithDarwinSdk "$path"; then
             skip "$path"
             $skipNext && n+=1
             continue

pkgs/build-support/wrapper-common/utils.bash

@@ -118,6 +118,19 @@ badPath() {
         "${p#"${TEMPDIR:-/tmp}"}" = "$p"
 }
 
+# Like `badPath`, but handles paths that may be interpreted relative to
+# `$SDKROOT` on Darwin. For example, `-L/usr/lib/swift` is interpreted
+# as `-L$SDKROOT/usr/lib/swift` when `$SDKROOT` is set and
+# `$SDKROOT/usr/lib/swift` exists.
+badPathWithDarwinSdk() {
+    sdkPath=$SDKROOT/$1
+    if [[ "@darwinMinVersion@" && -e $sdkPath ]]; then
+        badPath "$sdkPath"
+    else
+        badPath "$1"
+    fi
+}
+
 expandResponseParams() {
     declare -ga params=("$@")
     local arg