Fix WASM=0 warning in gen_struct_info (#8497)

Also, try to give more meaning errors when embuilder fails.

embuilder.py

@@ -11,6 +11,7 @@
 from __future__ import print_function
 import logging
 import os
+import subprocess
 import sys
 
 from tools import shared
@@ -147,11 +148,10 @@ def build(src, result_libs, args=[]):
   cpp = os.path.join(temp_dir, 'src.cpp')
   open(cpp, 'w').write(src)
   temp_js = os.path.join(temp_dir, 'out.js')
-  shared.Building.emcc(cpp, args, output_filename=temp_js)
-
-  # verify
-  if not os.path.exists(temp_js):
-    shared.exit_with_error('failed to build file')
+  try:
+    shared.Building.emcc(cpp, args, output_filename=temp_js)
+  except subprocess.CalledProcessError as e:
+    shared.exit_with_error("embuilder: emcc command failed with %d: '%s'", e.returncode, ' '.join(e.cmd))
 
   for lib in result_libs:
     if not os.path.exists(shared.Cache.get_path(lib)):

tools/gen_struct_info.py

@@ -388,7 +388,9 @@ def inspect_code(headers, cpp_opts, structs, defines):
   # Compile the program.
   show('Compiling generated code...')
   # -Oz optimizes enough to avoid warnings on code size/num locals
-  cmd = [shared.PYTHON, shared.EMCC] + cpp_opts + ['-o', js_file[1], src_file[1], '-s', 'BOOTSTRAPPING_STRUCT_INFO=1', '-s', 'WARN_ON_UNDEFINED_SYMBOLS=0', '-O0', '--js-opts', '0', '--memory-init-file', '0', '-s', 'SINGLE_FILE=1', '-s', 'WASM=0', '-Wno-format']
+  cmd = [shared.PYTHON, shared.EMCC] + cpp_opts + ['-o', js_file[1], src_file[1], '-s', 'BOOTSTRAPPING_STRUCT_INFO=1', '-s', 'WARN_ON_UNDEFINED_SYMBOLS=0', '-O0', '--js-opts', '0', '--memory-init-file', '0', '-s', 'SINGLE_FILE=1', '-Wno-format']
+  if not shared.Settings.WASM_BACKEND:
+    cmd += ['-s', 'WASM=0']
   if shared.Settings.WASM_OBJECT_FILES:
     cmd += ['-s', 'WASM_OBJECT_FILES=1']