depends: modernize clang flags

Fixes builds with llvm >= 11 in guix by working around the problem. As a bonus,
this is much cleaner and more maintainable than what we had before.

depends/hosts/darwin.mk

@@ -19,7 +19,6 @@ clang_prog=$(build_prefix)/bin/clang
 clangxx_prog=$(clang_prog)++
 llvm_config_prog=$(build_prefix)/bin/llvm-config
 
-clang_resource_dir=$(build_prefix)/lib/clang/$(native_clang_version)
 else
 # FORCE_USE_SYSTEM_CLANG is non-empty, so we use the clang from the user's
 # system
@@ -37,7 +36,6 @@ clang_prog=$(shell $(SHELL) $(.SHELLFLAGS) "command -v clang")
 clangxx_prog=$(shell $(SHELL) $(.SHELLFLAGS) "command -v clang++")
 llvm_config_prog=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-config")
 
-clang_resource_dir=$(shell clang -print-resource-dir)
 llvm_lib_dir=$(shell $(llvm_config_prog) --libdir)
 endif
 
@@ -63,53 +61,33 @@ $(foreach TOOL,$(cctools_TOOLS),$(eval darwin_$(TOOL) = $$(build_prefix)/bin/$$(
 #         Explicitly point to our binaries (e.g. cctools) so that they are
 #         ensured to be found and preferred over other possibilities.
 #
-#     -stdlib++-isystem$(OSX_SDK)/usr/include/c++/v1
+#     -isysroot$(OSX_SDK) -nostdlibinc
 #
-#         Forces clang to use the libc++ headers from our SDK and completely
-#         forget about the libc++ headers from the standard directories
+#         Disable default include paths built into the compiler as well as
+#         those normally included for libc and libc++. The only path that
+#         remains implicitly is the clang resource dir.
 #
-#     -Xclang -*system<path_a> \
-#     -Xclang -*system<path_b> \
-#     -Xclang -*system<path_c> ...
+#     -iwithsysroot / -iframeworkwithsysroot
 #
-#         Adds path_a, path_b, and path_c to the bottom of clang's list of
-#         include search paths. This is used to explicitly specify the list of
-#         system include search paths and its ordering, rather than rely on
-#         clang's autodetection routine. This routine has been shown to:
-#             1. Fail to pickup libc++ headers in $SYSROOT/usr/include/c++/v1
-#                when clang was built manually (see: https://github.com/bitcoin/bitcoin/pull/17919#issuecomment-656785034)
-#             2. Fail to pickup C headers in $SYSROOT/usr/include when
-#                C_INCLUDE_DIRS was specified at configure time (see: https://gist.github.com/dongcarl/5cdc6990b7599e8a5bf6d2a9c70e82f9)
-#
-#         Talking directly to cc1 with -Xclang here grants us access to specify
-#         more granular categories for these system include search paths, and we
-#         can use the correct categories that these search paths would have been
-#         placed in if the autodetection routine had worked correctly. (see:
-#         https://gist.github.com/dongcarl/5cdc6990b7599e8a5bf6d2a9c70e82f9#the-treatment)
-#
-#         Furthermore, it places these search paths after any "non-Xclang"
-#         specified search paths. This prevents any additional clang options or
-#         environment variables from coming after or in between these system
-#         include search paths, as that would be wrong in general but would also
-#         break #include_next's.
+#         Adds the desired paths from the SDK
 #
+
 darwin_CC=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH \
               -u OBJC_INCLUDE_PATH -u OBJCPLUS_INCLUDE_PATH -u CPATH \
               -u LIBRARY_PATH \
             $(clang_prog) --target=$(host) -mmacosx-version-min=$(OSX_MIN_VERSION) \
               -B$(build_prefix)/bin -mlinker-version=$(LD64_VERSION) \
-              -isysroot$(OSX_SDK) \
-              -Xclang -internal-externc-isystem -Xclang $(clang_resource_dir)/include \
-              -Xclang -internal-externc-isystem -Xclang $(OSX_SDK)/usr/include
+              -isysroot$(OSX_SDK) -nostdlibinc \
+              -iwithsysroot/usr/include -iframeworkwithsysroot/System/Library/Frameworks
+
 darwin_CXX=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH \
                -u OBJC_INCLUDE_PATH -u OBJCPLUS_INCLUDE_PATH -u CPATH \
                -u LIBRARY_PATH \
              $(clangxx_prog) --target=$(host) -mmacosx-version-min=$(OSX_MIN_VERSION) \
                -B$(build_prefix)/bin -mlinker-version=$(LD64_VERSION) \
-               -isysroot$(OSX_SDK) \
-               -stdlib++-isystem$(OSX_SDK)/usr/include/c++/v1 \
-               -Xclang -internal-externc-isystem -Xclang $(clang_resource_dir)/include \
-               -Xclang -internal-externc-isystem -Xclang $(OSX_SDK)/usr/include
+               -isysroot$(OSX_SDK) -nostdlibinc \
+               -iwithsysroot/usr/include/c++/v1 \
+               -iwithsysroot/usr/include -iframeworkwithsysroot/System/Library/Frameworks
 
 darwin_CFLAGS=-pipe -std=$(C_STANDARD)
 darwin_CXXFLAGS=-pipe -std=$(CXX_STANDARD)