Make the WASI_SDK_PATH variable mandatory when building for wasi host.
This requirement prevents developers from being stuck due to unfriendly
configuration's error message.
configure.ac: setup build tools and register objects
main.c: wrap main with rb_wasm_rt_start to handle asyncify unwinds
tool/m4/ruby_wasm_tools.m4: setup default command based on WASI_SDK_PATH
environment variable. checks wasm-opt which is used for asyncify.
tool/wasm-clangw wasm/wasm-opt: a clang wrapper which replaces real
wasm-opt with do-nothing wasm-opt to avoid misoptimization before
asyncify. asyncify is performed at POSTLINK, but clang linker driver
tries to run optimization by wasm-opt unconditionally. inlining pass
at wasm level breaks asyncify's assumption, so should not optimize
before POSTLIK.
wasm/GNUmakefile.in: wasm specific rules to compile objects
