Do support Windows again after requiring Rust

By default, Rust wants to build MS Visual C-compatible libraries on
Windows, because that is _the_ native C compiler.

Git is historically lacking in its MSVC support, and the official Git
for Windows versions are built using GCC instead. As a consequence, an
entire GCC toolchain is installed as part of the `windows-build` job of
every CI build.

Naturally, this requires adjustments in how Rust is called, most
importantly it requires installing support for a GCC-compatible build
target.

Let's make the necessary adjustment both in the CI-specific code that
installs Rust as well as in the Windows-specific configuration in
`config.mak.uname`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

ci/install-rust.sh

@@ -28,6 +28,9 @@ if [ "$BITNESS" = "32" ]; then
   $CARGO_HOME/bin/rustup default --force-non-host $RUST_VERSION || exit $?
 else
   $CARGO_HOME/bin/rustup default $RUST_VERSION || exit $?
+  if [ "$CI_OS_NAME" = "windows" ]; then
+    $CARGO_HOME/bin/rustup target add x86_64-pc-windows-gnu || exit $?
+  fi
 fi
 
 . $CARGO_HOME/env

config.mak.uname

@@ -727,19 +727,28 @@ ifeq ($(uname_S),MINGW)
 		prefix = /mingw32
 		HOST_CPU = i686
 		BASIC_LDFLAGS += -Wl,--pic-executable,-e,_mainCRTStartup
+		CARGO_BUILD_TARGET = i686-pc-windows-gnu
         endif
         ifeq (MINGW64,$(MSYSTEM))
 		prefix = /mingw64
 		HOST_CPU = x86_64
 		BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
+		CARGO_BUILD_TARGET = x86_64-pc-windows-gnu
         else ifeq (CLANGARM64,$(MSYSTEM))
 		prefix = /clangarm64
 		HOST_CPU = aarch64
 		BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
+		CARGO_BUILD_TARGET = aarch64-pc-windows-gnu
         else
 		COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
 		BASIC_LDFLAGS += -Wl,--large-address-aware
         endif
+
+	export CARGO_BUILD_TARGET
+	RUST_TARGET_DIR = rust/target/$(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 \
 		-fstack-protector-strong