Improve behavior for downloading build tools (#706)

* Add `-DBINDINGS_TARGET=OFF` and `-DUSE_WASM_COMPONENT_LD=OFF` to
disable downloading these programs entirely.
* Favor `wit-bindgen` on the system if found and if it has the right
version.
* Don't fail at configure-time if the arch or os of the host is unknown
since the tool may not be needed during the build.

Author: alexcrichton

cmake/ba-download.cmake

@@ -10,7 +10,8 @@ function(ba_download target repo version)
   elseif (CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "AMD64")
     set(arch "x86_64")
   else()
-    message(FATAL_ERROR "Unsupported architecture ${CMAKE_HOST_SYSTEM_PROCESSOR} for ${target}")
+    set(arch "UNKNOWN_ARCH")
+    message(WARNING "Unsupported architecture ${CMAKE_HOST_SYSTEM_PROCESSOR} for ${target}")
   endif()
 
   if (CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
@@ -20,7 +21,8 @@ function(ba_download target repo version)
   elseif (CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
     set(os windows)
   else()
-    message(FATAL_ERROR "Unsupported system ${CMAKE_HOST_SYSTEM_NAME} for ${target}")
+    set(os "UNKNOWN_OS")
+    message(WARNING "Unsupported system ${CMAKE_HOST_SYSTEM_NAME} for ${target}")
   endif()
 
   if (target STREQUAL wasmtime)

cmake/bindings.cmake

@@ -1,13 +1,40 @@
-include(ba-download)
+# Enable turning this rule off entirely if so desired.
+option(BINDINGS_TARGET "Generate bindings target" ON)
+if (NOT BINDINGS_TARGET)
+  return()
+endif()
 
-ba_download(
-  wit-bindgen
-  "https://github.com/bytecodealliance/wit-bindgen"
-  "0.48.0"
-)
-ExternalProject_Get_Property(wit-bindgen SOURCE_DIR)
-set(wit_bindgen "${SOURCE_DIR}/wit-bindgen")
+# If `wit-bindgen` is on the system and has the right version, favor that,
+# otherwise download a known good version.
+find_program(WIT_BINDGEN_EXECUTABLE NAMES wit-bindgen)
+if(WIT_BINDGEN_EXECUTABLE)
+  message(STATUS "Found wit-bindgen: ${WIT_BINDGEN_EXECUTABLE}")
+
+  execute_process(
+    COMMAND ${WIT_BINDGEN_EXECUTABLE} --version
+    OUTPUT_VARIABLE WIT_BINDGEN_VERSION
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+  if (NOT (WIT_BINDGEN_VERSION MATCHES "0\\.48\\.0"))
+    message(WARNING "wit-bindgen version 0.48.0 is required, found: ${WIT_BINDGEN_VERSION}")
+    set(WIT_BINDGEN_EXECUTABLE "")
+  endif()
+endif()
+
+if (NOT WIT_BINDGEN_EXECUTABLE)
+  include(ba-download)
+  ba_download(
+    wit-bindgen
+    "https://github.com/bytecodealliance/wit-bindgen"
+    "0.48.0"
+  )
+  ExternalProject_Get_Property(wit-bindgen SOURCE_DIR)
+  set(wit_bindgen "${SOURCE_DIR}/wit-bindgen")
+else()
+  set(wit_bindgen ${WIT_BINDGEN_EXECUTABLE})
+endif()
 
+include(ExternalProject)
 ExternalProject_Add(
   wasi-wits
   URL https://github.com/WebAssembly/wasi-cli/archive/refs/tags/v0.2.0.tar.gz

cmake/wasm-component-ld.cmake

@@ -1,7 +1,12 @@
+option(USE_WASM_COMPONENT_LD "Use wasm-component-ld as the linker for WebAssembly components" ON)
+if (NOT USE_WASM_COMPONENT_LD)
+  return()
+endif()
+
 find_program(WASM_COMPONENT_LD_EXECUTABLE NAMES wasm-component-ld)
-include(ba-download)
 
 if (NOT WASM_COMPONENT_LD_EXECUTABLE)
+  include(ba-download)
   ba_download(
     wasm-component-ld
     "https://github.com/bytecodealliance/wasm-component-ld"