depends: specify libc++ header location for darwin

For depends builds this was fixed by fbcfcf6, which deleted the conflicting
headers. When we no longer control the clang installation, we need to ensure
that the SDK's libc++ headers are used rather than the ones shipped with clang.
We can do that by turning off the default include path and hard-coding our own.
This hard-coded path is ok because we control (via SDK packaging) where these
headers end-up.

Side-note: Now that this path is hard-coded in depends, we can potentially
package the SDK differently, as the c++ folder can live wherever is most
convenient for us.

Author: theuni

depends/hosts/darwin.mk

@@ -9,7 +9,7 @@ OSX_SDK=$(SDK_PATH)/Xcode-$(XCODE_VERSION)-$(XCODE_BUILD_ID)-extracted-SDK-with-
 # When cross-compiling for Darwin using Clang, -mlinker-version must be passed to
 # ensure that modern linker features are enabled.
 darwin_CC=clang -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -mlinker-version=$(LD64_VERSION) -B$(build_prefix)/bin
-darwin_CXX=clang++ -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -stdlib=libc++ -mlinker-version=$(LD64_VERSION) -B$(build_prefix)/bin
+darwin_CXX=clang++ -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -stdlib=libc++ -mlinker-version=$(LD64_VERSION) -B$(build_prefix)/bin -nostdinc++ -isystem $(OSX_SDK)/usr/include/c++/v1
 
 darwin_CFLAGS=-pipe
 darwin_CXXFLAGS=$(darwin_CFLAGS)