diff --git a/lib-src/lua/Makefile.am b/lib-src/lua/Makefile.am
index 48e0af32c..6b064d8f0 100644
--- a/lib-src/lua/Makefile.am
+++ b/lib-src/lua/Makefile.am
@@ -25,5 +25,5 @@ liblua_a_CFLAGS += -DLUA_USE_LINUX
 endif
 
 if CXXLUA
-liblua_a_CFLAGS += -x c++ $(AM_CXXFLAGS) $(CXXFLAGS)
+liblua_a_CFLAGS = -x c++ $(AM_CXXFLAGS) $(CXXFLAGS)
 endif
diff --git a/tools/Makefile.am b/tools/Makefile.am
index 226781223..e04a8b774 100644
--- a/tools/Makefile.am
+++ b/tools/Makefile.am
@@ -21,5 +21,5 @@ tolua_CFLAGS = -I$(top_srcdir)/lib-src/lua $(AM_CFLAGS)
 tolua_LDADD = $(top_builddir)/lib-src/lua/liblua.a $(AM_LDADD)
 
 if CXXLUA
-tolua_CFLAGS += -x c++ $(AM_CXXFLAGS) $(CXXFLAGS)
+tolua_CFLAGS = -I$(top_srcdir)/lib-src/lua -x c++ $(AM_CXXFLAGS) $(CXXFLAGS)
 endif