win+Meson: do allow linking with the Rust-built xdiff

When linking against the Rust-built `xdiff`, there is now a new required
dependency: Without _also_ linking to the system library `userenv`, the
compile would fail with this error message:

  xdiff.lib(std-c85e9beb7923f636.std.df32d1bc89881d89-cgu.0.rcgu.o) :
  error LNK2019: unresolved external symbol __imp_GetUserProfileDirectoryW
  referenced in function _ZN3std3env8home_dir17hfd1c3b6676cd78f6E

Therefore, just like we do in case of Makefile-based builds on Windows,
we now also link to that library when building with Meson.

Note that if we only have Rust depend upon libuserenv then at link time
GCC would complain about:

  undefined reference to `GetUserProfileDirectoryW'

Apparently there is _some_ closure that gets compiled in that requires
this function, and that in turn forces Git to link to libuserenv.

This is a new requirement, and therefore has not been made part of the
"minimal Git for Windows SDK".

In the near future, I intend to include it, but for now let's just
ensure that the file is added manually if it is missing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
[en: Squashed a few of Johannes's patches, and moved lib userenv
 handling from an earlier patch]
Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dscho

.github/workflows/main.yml

@@ -115,6 +115,14 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
+    - name: ensure that libuserenv.a is present
+      shell: bash
+      run: |
+        cd /mingw64/lib && {
+          test -f libuserenv.a ||
+          /c/Program\ Files/Git/mingw64/bin/curl -Lo libuserenv.a \
+            https://github.com/git-for-windows/git-sdk-64/raw/HEAD/mingw64/lib/libuserenv.a
+        }
     - name: build
       shell: bash
       env:

config.mak.uname

@@ -765,6 +765,8 @@ ifeq ($(uname_S),MINGW)
 
 	export CARGO_BUILD_TARGET
 	RUST_TARGET_DIR = rust/target/$(CARGO_BUILD_TARGET)/$(RUST_BUILD_MODE)
+	# Unfortunately now needed because of Rust
+	EXTLIBS += -luserenv
 
 	CC = gcc
 	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY \

meson.build

@@ -1293,6 +1293,7 @@ elif host_machine.system() == 'windows'
   ]
 
   libgit_dependencies += compiler.find_library('ntdll')
+  libgit_dependencies += compiler.find_library('userenv')
   libgit_include_directories += 'compat/win32'
   if compiler.get_id() == 'msvc'
     libgit_include_directories += 'compat/vcbuild/include'