[Bug #19967] Reset `LIBPATHENV` env after started

Not to affect other tools invoked as child processes.

common.mk

@@ -863,7 +863,8 @@ $(arch:noarch=ignore)-fake.rb: $(srcdir)/template/fake.rb.in $(tooldir)/generic_
 	$(ECHO) generating $@
 	$(Q) $(CPP) -DRUBY_EXPORT $(INCFLAGS) $(CPPFLAGS) "$(srcdir)/version.c" | \
 	$(BOOTSTRAPRUBY) "$(tooldir)/generic_erb.rb" -o $@ "$(srcdir)/template/fake.rb.in" \
-		i=- srcdir="$(srcdir)" BASERUBY="$(BASERUBY)"
+	    i=- srcdir="$(srcdir)" BASERUBY="$(BASERUBY)" \
+	    LIBPATHENV="$(LIBPATHENV)" PRELOADENV="$(PRELOADENV)" LIBRUBY_SO="$(LIBRUBY_SO)"
 
 noarch-fake.rb: # prerequisite of yes-fake
 	$(Q) exit > $@

template/Makefile.in

@@ -127,6 +127,9 @@ BUILTIN_TRANSSRCS = @BUILTIN_TRANSSRCS@
 BUILTIN_TRANSOBJS = @BUILTIN_TRANSOBJS@
 POSTLINK = @POSTLINK@
 
+LIBPATHENV = @LIBPATHENV@
+PRELOADENV = @PRELOADENV@
+
 RUBY_BASE_NAME=@RUBY_BASE_NAME@
 RUBY_API_VERSION=@RUBY_API_VERSION@
 RUBY_INSTALL_NAME=@RUBY_INSTALL_NAME@

template/fake.rb.in

@@ -52,6 +52,9 @@ class Object
 % }
 end
 builddir = File.dirname(File.expand_path(__FILE__))
+libpathenv = libpathenv = "<%=arg['LIBPATHENV']%>"
+preloadenv = preloadenv = "<%=arg['PRELOADENV']%>"
+libruby_so = libruby_so = "<%=arg['LIBRUBY_SO']%>"
 srcdir = "<%=arg['srcdir']%>"
 top_srcdir = File.realpath(srcdir, builddir)
 fake = File.join(top_srcdir, "tool/fake.rb")

tool/fake.rb

@@ -9,6 +9,14 @@ class File
   end
 end
 
+[[libpathenv, "."], [preloadenv, libruby_so]].each do |env, path|
+  env or next
+  e = ENV[env] or next
+  e = e.split(File::PATH_SEPARATOR)
+  e.delete(File.expand_path(path, builddir)) or next
+  ENV[env] = (e.join(File::PATH_SEPARATOR) unless e.empty?)
+end
+
 static = !!(defined?($static) && $static)
 $:.unshift(builddir)
 posthook = proc do