diff --git a/src/parser/xhpast/bin/PhutilXHPASTBinary.php b/src/parser/xhpast/bin/PhutilXHPASTBinary.php
--- a/src/parser/xhpast/bin/PhutilXHPASTBinary.php
+++ b/src/parser/xhpast/bin/PhutilXHPASTBinary.php
@@ -40,7 +40,7 @@
       '%s -C %s %Ls',
       $command,
       $path,
-      array('clean', 'all', 'install'));
+      array('SKIP_PARSER=1', 'SKIP_SCANNER=1', 'clean', 'all', 'install'));
 
     // Test the binary.
     if (!self::isAvailable()) {
diff --git a/support/xhpast/Makefile b/support/xhpast/Makefile
--- a/support/xhpast/Makefile
+++ b/support/xhpast/Makefile
@@ -39,18 +39,27 @@
 	cp xhpast $(ROOT)/bin/xhpast
 
 .PHONY: parser scanner
+
 parser: parser.yacc.hpp parser.yacc.cpp
 scanner: scanner.lex.hpp scanner.lex.cpp
 
 %.lex.hpp %.lex.cpp: %.l
+ifndef SKIP_SCANNER
 	flex $(FLEXFLAGS) --header-file=$*.lex.hpp --outfile=$*.lex.cpp $<
 	@echo '/* @gen''er''ated */' >> $*.lex.hpp
 	@echo '/* @gen''er''ated */' >> $*.lex.cpp
+else
+	touch $*.lex.hpp $*.lex.cpp
+endif
 
 %.yacc.hpp %.yacc.cpp: %.y
+ifndef SKIP_PARSER
 	bison $(BISONFLAGS) --defines=$*.yacc.hpp --output=$*.yacc.cpp $<
 	@echo '/* @gen''er''ated */' >> $*.yacc.hpp
 	@echo '/* @gen''er''ated */' >> $*.yacc.cpp
+else
+	touch $*.yacc.hpp $*.yacc.cpp
+endif
 
 %.o: %.cpp
 	$(CXX) -c $(CPPFLAGS) -o $@ $<