fixup! Fix dump_ast build for cross-compiling

Author: nobu

tool/dump_ast.mkmf.rb

@@ -10,14 +10,15 @@
 target = src.basename.sub_ext('')
 dirs = objs.map {|obj| File.dirname(obj)}.uniq - %w[.]
 link = MakeMakefile::TRY_LINK.sub(MakeMakefile::CONFTEST+$EXEEXT, '$(@)')
+prismdir= "$(srcdir)/#{dirs.first}"
+$VPATH = ["$(srcdir)", "$(srcdir)/#{tooldir.basename}", prismdir, tooldir]
+$INCFLAGS << " -I#{prismdir}"
+$CPPFLAGS = $CFLAGS = $INCFLAGS
 
 include FileUtils::Verbose
 mkpath(workdir)
 Dir.chdir(workdir) {
   mkpath(dirs)
-  prismdir= "$(srcdir)/#{dirs.first}"
-  $VPATH = ["$(srcdir)", "$(srcdir)/#{tooldir.basename}", prismdir, tooldir]
-  $INCFLAGS << " -I#{prismdir}"
   File.write('Makefile', [MakeMakefile.configuration(srcdir.to_s), <<~MAKEFILE].join(""))
     target = #{target}#{$EXEEXT}
     objs = #{objs.join(' ')}