update default host to use clang

depends/hosts/default.mk

@@ -1,17 +1,36 @@
-default_host_CC = $(host_toolchain)gcc
-default_host_CXX = $(host_toolchain)g++
-default_host_AR = $(host_toolchain)ar
-default_host_RANLIB = $(host_toolchain)ranlib
-default_host_STRIP = $(host_toolchain)strip
+# Flag explanations:
+#
+#     -B$(build_prefix)/bin
+#
+#         Explicitly point to our binaries (e.g. cctools) so that they are
+#         ensured to be found and preferred over other possibilities.
+#
+default_host_CC = clang -target $(host) -B$(build_prefix)/bin
+default_host_CXX = clang++ -target $(host) -B$(build_prefix)/bin -stdlib=libc++
+default_host_AR = llvm-ar
+default_host_RANLIB = llvm-ranlib
+default_host_STRIP = llvm-strip
 default_host_LIBTOOL = $(host_toolchain)libtool
 default_host_INSTALL_NAME_TOOL = $(host_toolchain)install_name_tool
 default_host_OTOOL = $(host_toolchain)otool
-default_host_NM = $(host_toolchain)nm
+default_host_NM = llvm-nm
+
+$(host_os)_native_binutils?=native_clang
+$(host_os)_native_toolchain?=native_clang
 
 define add_host_tool_func
+ifneq ($(filter $(origin $1),undefined default),)
+# Do not consider the well-known var $1 if it is undefined or is taking a value
+# that is predefined by "make" (e.g. the make variable "CC" has a predefined
+# value of "cc")
 $(host_os)_$1?=$$(default_host_$1)
 $(host_arch)_$(host_os)_$1?=$$($(host_os)_$1)
 $(host_arch)_$(host_os)_$(release_type)_$1?=$$($(host_os)_$1)
+else
+$(host_os)_$1=$(or $($1),$($(host_os)_$1),$(default_host_$1))
+$(host_arch)_$(host_os)_$1=$(or $($1),$($(host_arch)_$(host_os)_$1),$$($(host_os)_$1))
+$(host_arch)_$(host_os)_$(release_type)_$1=$(or $($1),$($(host_arch)_$(host_os)_$(release_type)_$1),$$($(host_os)_$1))
+endif
 host_$1=$$($(host_arch)_$(host_os)_$1)
 endef