feat: 声明自己的 std 模块源

构建工具否则会向编译器问 std.cppm 在哪,而这一份是这个包为一个编译器毫不知情的
目标配置出来的:它的 __config_site 在 llvm-generated/,它的片段在
llvm/libcxx/modules/std/。

三个 flag 都是实测逼出来的,不是预防性的:
  --no-default-config  载荷的 clang++.cfg 无条件加一个宿主 C 库的头,
                       模块于是编那个库的 <wchar.h> 并停在它指望宿主编译器
                       提供的名字上
  -nostdinc/-nostdinc++ 同上
  -D_GNU_SOURCE        libc++ 的 locale 层去够 musl 只在这个宏下声明的名字
                       (vasprintf、strtof_l 等),LLVM 自己的 runtimes 构建
                       出于同样理由定义它
  -I llvm/libcxx/modules  模块源按相对名 include 九十来个片段,它们是上游的、
                       未改动,所以留在上游放的地方,由搜索路径点名那个目录

配套:mcpp-community/mcpp#486。

Author: speak-agent

mcpp.toml

@@ -17,6 +17,38 @@ repo        = "https://github.com/mcpplibs/openkal-llvm-runtime"
 # declared here and read there.
 provides = ["hosted-standard-library"]
 
+# Where this package's `std' module source is, and what it needs.
+#
+# The build tool otherwise asks the compiler (`-print-library-module-manifest-path'),
+# which is the right question when the standard library is the compiler's own
+# and the wrong one here: this source was configured for a target the compiler
+# knows nothing about, and its configuration is in llvm-generated/ rather than
+# beside the compiler.
+std-module = "llvm-generated/std.cppm"
+std-module-flags = [
+  # ⚠️ --no-default-config, and it is not tidiness.
+  #
+  # The toolchain payload ships a clang++.cfg that names a host C library's
+  # headers unconditionally. For an ordinary translation unit that is right and
+  # invisible; for this one it puts a C library that is not the one beneath this
+  # package first, and the module fails compiling <wchar.h> with names that
+  # library expects the host compiler to have supplied. Measured.
+  "--no-default-config",
+  "-nostdinc",
+  "-nostdinc++",
+  # libc++'s locale layer reaches for names musl declares only under this macro
+  # (vasprintf, strtof_l and their relatives). LLVM's own runtimes build defines
+  # it for the same reason. It is stated here rather than left to the target
+  # block, because that block describes how this package's own sources are
+  # compiled and the module is compiled separately.
+  "-D_GNU_SOURCE",
+  # The module source is one file that includes ninety-odd fragments by
+  # relative name. They are upstream's and unmodified, so they stay where
+  # upstream put them and the search path names that directory rather than the
+  # fragments being copied next to the configured source.
+  "-I", "llvm/libcxx/modules",
+]
+
 # The C library beneath. A C++ standard library is not portable in the sense a
 # program is: it is configured for one C library and compiled against that
 # library's headers, and the configuration is what this package supplies. The