Update for darwin

depends/builders/darwin.mk

@@ -1,5 +1,5 @@
-build_darwin_CC = gcc-5
-build_darwin_CXX = g++-5
+build_darwin_CC: = $(shell xcrun -f clang)
+build_darwin_CXX: = $(shell xcrun -f clang++)
 build_darwin_AR: = $(shell xcrun -f ar)
 build_darwin_RANLIB: = $(shell xcrun -f ranlib)
 build_darwin_STRIP: = $(shell xcrun -f strip)
@@ -10,8 +10,8 @@ build_darwin_SHA256SUM = shasum -a 256
 build_darwin_DOWNLOAD = curl --location --fail --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o
 
 #darwin host on darwin builder. overrides darwin host preferences.
-darwin_CC= gcc-5
-darwin_CXX= g++-5
+darwin_CC=$(shell xcrun -f clang) -mmacosx-version-min=$(OSX_MIN_VERSION)
+darwin_CXX:=$(shell xcrun -f clang++) -mmacosx-version-min=$(OSX_MIN_VERSION) -stdlib=libc++
 darwin_AR:=$(shell xcrun -f ar)
 darwin_RANLIB:=$(shell xcrun -f ranlib)
 darwin_STRIP:=$(shell xcrun -f strip)
@@ -19,4 +19,4 @@ darwin_LIBTOOL:=$(shell xcrun -f libtool)
 darwin_OTOOL:=$(shell xcrun -f otool)
 darwin_NM:=$(shell xcrun -f nm)
 darwin_INSTALL_NAME_TOOL:=$(shell xcrun -f install_name_tool)
-darwin_native_toolchain=
+darwin_native_toolchain=