Make build work on Windows

Makefile

@@ -3,10 +3,24 @@
 
 ROOT_DIR=${CURDIR}
 LLVM_PROJ_DIR?=$(ROOT_DIR)/src/llvm-project
+
+# msys needs any /-prefixed arguments, or =/ containing, to turn into //
+# because it tries to path-expand the / into the msys root.  // escapes this.
+SLASHY_SLASH=
+
+ifeq ($(OS),Windows_NT)
+PREFIX?=c:/wasi-sdk
+BASH?=bash -c
+ifneq (x$(MSYSTEM),x)
+SLASHY_SLASH=/
+endif
+else
 PREFIX?=/opt/wasi-sdk
+BASH?=
+endif
 
-CLANG_VERSION=$(shell ./llvm_version.sh $(LLVM_PROJ_DIR))
-VERSION:=$(shell ./version.sh)
+CLANG_VERSION=$(shell $(BASH) ./llvm_version.sh $(LLVM_PROJ_DIR))
+VERSION:=$(shell $(BASH) ./version.sh)
 DEBUG_PREFIX_MAP=-fdebug-prefix-map=$(ROOT_DIR)=wasisdk://v$(VERSION)
 
 default: build
@@ -109,7 +123,7 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 	cd build/libcxx && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS) \
 	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP)" \
 	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP)" \
-	    -DLIBCXX_LIBDIR_SUFFIX=/wasm32-wasi \
+	    -DLIBCXX_LIBDIR_SUFFIX=$(SLASHY_SLASH)/wasm32-wasi \
 	    $(LLVM_PROJ_DIR)/libcxx
 	ninja $(NINJA_FLAGS) -v -C build/libcxx
 	# Do the install.
@@ -147,7 +161,7 @@ build/libcxxabi.BUILT: build/libcxx.BUILT build/llvm.BUILT
 	cd build/libcxxabi && cmake -G Ninja $(LIBCXXABI_CMAKE_FLAGS) \
 	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP)" \
 	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP)" \
-	    -DLIBCXXABI_LIBDIR_SUFFIX=/wasm32-wasi \
+	    -DLIBCXXABI_LIBDIR_SUFFIX=$(SLASHY_SLASH)/wasm32-wasi \
 	    $(LLVM_PROJ_DIR)/libcxxabi
 	ninja $(NINJA_FLAGS) -v -C build/libcxxabi
 	# Do the install.

wasi-sdk.cmake

@@ -8,10 +8,16 @@ set(CMAKE_SYSTEM_VERSION 1)
 set(CMAKE_SYSTEM_PROCESSOR wasm32)
 set(triple wasm32-wasi)
 
-set(CMAKE_C_COMPILER ${WASI_SDK_PREFIX}/bin/clang)
-set(CMAKE_CXX_COMPILER ${WASI_SDK_PREFIX}/bin/clang++)
-set(CMAKE_AR ${WASI_SDK_PREFIX}/bin/llvm-ar CACHE STRING "wasi-sdk build")
-set(CMAKE_RANLIB ${WASI_SDK_PREFIX}/bin/llvm-ranlib CACHE STRING "wasi-sdk build")
+if(WIN32)
+	set(WASI_HOST_EXE_SUFFIX ".exe")
+else()
+	set(WASI_HOST_EXE_SUFFIX "")
+endif()
+
+set(CMAKE_C_COMPILER ${WASI_SDK_PREFIX}/bin/clang${WASI_HOST_EXE_SUFFIX})
+set(CMAKE_CXX_COMPILER ${WASI_SDK_PREFIX}/bin/clang++${WASI_HOST_EXE_SUFFIX})
+set(CMAKE_AR ${WASI_SDK_PREFIX}/bin/llvm-ar${WASI_HOST_EXE_SUFFIX} CACHE STRING "wasi-sdk build")
+set(CMAKE_RANLIB ${WASI_SDK_PREFIX}/bin/llvm-ranlib${WASI_HOST_EXE_SUFFIX} CACHE STRING "wasi-sdk build")
 set(CMAKE_C_COMPILER_TARGET ${triple} CACHE STRING "wasi-sdk build")
 set(CMAKE_CXX_COMPILER_TARGET ${triple} CACHE STRING "wasi-sdk build")
 set(CMAKE_C_FLAGS "-v" CACHE STRING "wasi-sdk build")