[ion] Modularize the HTML simulator

build/rules.mk

@@ -31,6 +31,11 @@ $(eval $(call rule_for, \
   $$(CXX) $$(CXXFLAGS) $$(SFLAGS) -c $$< -o $$@ \
 ))
 
+$(eval $(call rule_for, \
+  CPP, %, %.inc, \
+  $$(CPP) -P $$< $$@ \
+))
+
 ifeq ($(OS),Windows_NT)
 # Work around command-line length limit
 # On Msys2 the max command line is 32 000 characters. Our standard LD command

build/toolchain.emscripten.mak

@@ -1,6 +1,7 @@
 CC = emcc
 CXX = emcc
 LD = emcc
+CPP = cpp
 
 EMSCRIPTEN_ASYNC_SYMBOLS = \
 SAFE_HEAP_LOAD \