Update how wasi-sdk uses cmake

Change `CMAKE_SYSTEM_NAME` to `WASI`, use a `CMAKE_MODULE_PATH`,
remove `CACHE` usage, and fix the compiler autodetection code so that
the "compiler works" hacks are no longer needed, following the advice in
[this comment](https://gitlab.kitware.com/cmake/cmake/-/issues/19223#note_567327).

This is hopefully a step towards re-submitting WASI support to upstream cmake.

Makefile

@@ -55,6 +55,7 @@ clean:
 build/llvm.BUILT:
 	mkdir -p build/llvm
 	cd build/llvm && cmake -G Ninja \
+		-DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
 		-DCMAKE_BUILD_TYPE=MinSizeRel \
 		-DCMAKE_INSTALL_PREFIX=$(PREFIX) \
 		-DLLVM_TARGETS_TO_BUILD=WebAssembly \
@@ -92,9 +93,14 @@ build/wasi-libc.BUILT: build/llvm.BUILT
 	touch build/wasi-libc.BUILT
 
 build/compiler-rt.BUILT: build/llvm.BUILT
+	# Create an empty library so that cmake's configure checks which
+	# run the compiler to see if it produces working binaries succeed.
+	mkdir -p  $(PREFIX)/lib/clang/$(CLANG_VERSION)/lib/wasi
+	$(AR) cr $(PREFIX)/lib/clang/$(CLANG_VERSION)/lib/wasi/libclang_rt.builtins-wasm32.a
 	# Do the build, and install it.
 	mkdir -p build/compiler-rt
 	cd build/compiler-rt && cmake -G Ninja \
+		-DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
 		-DCMAKE_BUILD_TYPE=RelWithDebInfo \
 		-DCMAKE_TOOLCHAIN_FILE=$(ROOT_DIR)/wasi-sdk.cmake \
 		-DCOMPILER_RT_BAREMETAL_BUILD=On \
@@ -117,6 +123,7 @@ build/compiler-rt.BUILT: build/llvm.BUILT
 
 # Flags for libcxx.
 LIBCXX_CMAKE_FLAGS = \
+    -DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
     -DCMAKE_TOOLCHAIN_FILE=$(ROOT_DIR)/wasi-sdk.cmake \
     -DCMAKE_STAGING_PREFIX=$(PREFIX)/share/wasi-sysroot \
     -DLLVM_CONFIG_PATH=$(ROOT_DIR)/build/llvm/bin/llvm-config \
@@ -138,9 +145,12 @@ LIBCXX_CMAKE_FLAGS = \
     -DWASI_SDK_PREFIX=$(BUILD_PREFIX) \
     --debug-trycompile
 
-    #-DCMAKE_STAGING_PREFIX= 
-
 build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUILT
+	# Create an empty library so that cmake's configure checks which
+	# run the compiler to see if it produces working binaries succeed.
+	mkdir -p  $(PREFIX)/lib/clang/$(CLANG_VERSION)/lib/wasi
+	$(AR) cr $(PREFIX)/share/wasi-sysroot/lib/wasm32-wasi/libc++.a
+	$(AR) cr $(PREFIX)/share/wasi-sysroot/lib/wasm32-wasi/libc++abi.a
 	# Do the build.
 	mkdir -p build/libcxx
 	cd build/libcxx && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS) \
@@ -155,6 +165,7 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 
 # Flags for libcxxabi.
 LIBCXXABI_CMAKE_FLAGS = \
+    -DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
     -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON \
     -DCMAKE_CXX_COMPILER_WORKS=ON \
     -DCMAKE_C_COMPILER_WORKS=ON \
@@ -180,6 +191,10 @@ LIBCXXABI_CMAKE_FLAGS = \
     --debug-trycompile
 
 build/libcxxabi.BUILT: build/libcxx.BUILT build/llvm.BUILT
+	# Create an empty library so that cmake's configure checks which
+	# run the compiler to see if it produces working binaries succeed.
+	mkdir -p  $(PREFIX)/lib/clang/$(CLANG_VERSION)/lib/wasi
+	$(AR) cr $(PREFIX)/share/wasi-sysroot/lib/wasm32-wasi/libc++.a
 	# Do the build.
 	mkdir -p build/libcxxabi
 	cd build/libcxxabi && cmake -G Ninja $(LIBCXXABI_CMAKE_FLAGS) \

cmake/Platform/WASI.cmake

@@ -0,0 +1 @@
+set(WASI 1)

wasi-sdk.cmake

@@ -3,7 +3,7 @@
 # This is arbitrary, AFAIK, for now.
 cmake_minimum_required(VERSION 3.4.0)
 
-set(CMAKE_SYSTEM_NAME Wasm)
+set(CMAKE_SYSTEM_NAME WASI)
 set(CMAKE_SYSTEM_VERSION 1)
 set(CMAKE_SYSTEM_PROCESSOR wasm32)
 set(triple wasm32-wasi)
@@ -16,15 +16,10 @@ 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")
-set(CMAKE_CXX_FLAGS "-v -std=c++11" CACHE STRING "wasi-sdk build")
-
-set(CMAKE_SYSROOT ${WASI_SDK_PREFIX}/share/wasi-sysroot CACHE STRING "wasi-sdk build")
-set(CMAKE_STAGING_PREFIX ${WASI_SDK_PREFIX}/share/wasi-sysroot CACHE STRING "wasi-sdk build")
+set(CMAKE_AR ${WASI_SDK_PREFIX}/bin/llvm-ar${WASI_HOST_EXE_SUFFIX})
+set(CMAKE_RANLIB ${WASI_SDK_PREFIX}/bin/llvm-ranlib${WASI_HOST_EXE_SUFFIX})
+set(CMAKE_C_COMPILER_TARGET ${triple})
+set(CMAKE_CXX_COMPILER_TARGET ${triple})
 
 # Don't look in the sysroot for executables to run during the build
 set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
@@ -32,7 +27,3 @@ set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
-
-# Some other hacks
-set(CMAKE_C_COMPILER_WORKS ON)
-set(CMAKE_CXX_COMPILER_WORKS ON)