darwin-frameworks: add hook for private frameworks to CoreMIDI

Some parts of CoreMIDI depend on private frameworks, this enables eg.
adding -framework MultitouchSupport to a build.

pkgs/os-specific/darwin/apple-sdk/private-frameworks-setup-hook.sh

@@ -0,0 +1,8 @@
+addPrivateFrameworks() {
+    flag="-F/System/Library/PrivateFrameworks"
+    if [[ "$NIX_CFLAGS_COMPILE" != *$flag* ]]; then
+        NIX_CFLAGS_COMPILE+=" $flag"
+    fi
+}
+
+addEnvHooks "$hostOffset" addPrivateFrameworks