Update how wasi-sdk uses cmake (#161)

* 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.

* Add --sysroot to compiler-rt's C flags.

This ensures that it finds the built sysroot when testing whether the C
compiler works.

* Fixes.

* Use the just-built ar rather than the host ar.

* Use -DCMAKE_C_COMPILER_WORKS=ON instead of stubbing out archive files.

* Use -DCMAKE_CXX_COMPILER_WORKS=ON too.

* Restore the `--sysroot` argument when building compiler-rt.

compiler-rt depends on some libc headers, so it needs a sysroot.

* Add sysroot arguments for libc++ and libc++abi too.

* Also set CMAKE_AR.

It appears with the removal of the AR checks cmake is no longer
autodetecting CMAKE_AR, so set it explicitly.

Makefile

@@ -95,6 +95,10 @@ build/compiler-rt.BUILT: build/llvm.BUILT
 	# Do the build, and install it.
 	mkdir -p build/compiler-rt
 	cd build/compiler-rt && cmake -G Ninja \
+		-DCMAKE_C_COMPILER_WORKS=ON \
+		-DCMAKE_CXX_COMPILER_WORKS=ON \
+		-DCMAKE_AR=$(BUILD_PREFIX)/bin/ar \
+		-DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
 		-DCMAKE_BUILD_TYPE=RelWithDebInfo \
 		-DCMAKE_TOOLCHAIN_FILE=$(ROOT_DIR)/wasi-sdk.cmake \
 		-DCOMPILER_RT_BAREMETAL_BUILD=On \
@@ -104,7 +108,7 @@ build/compiler-rt.BUILT: build/llvm.BUILT
 		-DCOMPILER_RT_ENABLE_IOS=OFF \
 		-DCOMPILER_RT_DEFAULT_TARGET_ONLY=On \
 		-DWASI_SDK_PREFIX=$(BUILD_PREFIX) \
-		-DCMAKE_C_FLAGS="-O1 $(DEBUG_PREFIX_MAP)" \
+		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
 		-DLLVM_CONFIG_PATH=$(ROOT_DIR)/build/llvm/bin/llvm-config \
 		-DCOMPILER_RT_OS_DIR=wasi \
 		-DCMAKE_INSTALL_PREFIX=$(PREFIX)/lib/clang/$(CLANG_VERSION)/ \
@@ -117,6 +121,10 @@ build/compiler-rt.BUILT: build/llvm.BUILT
 
 # Flags for libcxx.
 LIBCXX_CMAKE_FLAGS = \
+    -DCMAKE_C_COMPILER_WORKS=ON \
+    -DCMAKE_CXX_COMPILER_WORKS=ON \
+    -DCMAKE_AR=$(BUILD_PREFIX)/bin/ar \
+    -DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
     -DCMAKE_TOOLCHAIN_FILE=$(ROOT_DIR)/wasi-sdk.cmake \
     -DCMAKE_STAGING_PREFIX=$(PREFIX)/share/wasi-sysroot \
     -DLLVM_CONFIG_PATH=$(ROOT_DIR)/build/llvm/bin/llvm-config \
@@ -138,14 +146,12 @@ LIBCXX_CMAKE_FLAGS = \
     -DWASI_SDK_PREFIX=$(BUILD_PREFIX) \
     --debug-trycompile
 
-    #-DCMAKE_STAGING_PREFIX= 
-
 build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUILT
 	# Do the build.
 	mkdir -p build/libcxx
 	cd build/libcxx && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS) \
-	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP)" \
-	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP)" \
+	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
+	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
 	    -DLIBCXX_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi \
 	    $(LLVM_PROJ_DIR)/libcxx
 	ninja $(NINJA_FLAGS) -v -C build/libcxx
@@ -155,9 +161,11 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 
 # Flags for libcxxabi.
 LIBCXXABI_CMAKE_FLAGS = \
-    -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON \
-    -DCMAKE_CXX_COMPILER_WORKS=ON \
     -DCMAKE_C_COMPILER_WORKS=ON \
+    -DCMAKE_CXX_COMPILER_WORKS=ON \
+    -DCMAKE_AR=$(BUILD_PREFIX)/bin/ar \
+    -DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
+    -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON \
     -DLIBCXXABI_ENABLE_EXCEPTIONS:BOOL=OFF \
     -DLIBCXXABI_ENABLE_SHARED:BOOL=OFF \
     -DLIBCXXABI_SILENT_TERMINATE:BOOL=ON \
@@ -183,8 +191,8 @@ build/libcxxabi.BUILT: build/libcxx.BUILT build/llvm.BUILT
 	# Do the build.
 	mkdir -p build/libcxxabi
 	cd build/libcxxabi && cmake -G Ninja $(LIBCXXABI_CMAKE_FLAGS) \
-	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP)" \
-	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP)" \
+	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
+	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
 	    -DLIBCXXABI_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi \
 	    $(LLVM_PROJ_DIR)/libcxxabi
 	ninja $(NINJA_FLAGS) -v -C build/libcxxabi

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)