Do not use -isystem for modules dir

Fixes #567

source/backend/Makefile.mk

@@ -24,7 +24,7 @@ endif
 
 # ----------------------------------------------------------------------------------------------------------------------------
 
-BUILD_CXX_FLAGS += -I. -I.. -I$(CWD) -I$(CWD)/includes -I$(CWD)/utils -isystem $(CWD)/modules
+BUILD_CXX_FLAGS += -I. -I.. -I$(CWD) -I$(CWD)/includes -I$(CWD)/modules -I$(CWD)/utils
 BUILD_CXX_FLAGS += $(LIBLO_FLAGS)
 
 # ----------------------------------------------------------------------------------------------------------------------------

source/bridges-plugin/Makefile

@@ -22,7 +22,7 @@ endif
 
 # ----------------------------------------------------------------------------------------------------------------------
 
-BUILD_CXX_FLAGS += -DBUILD_BRIDGE -I. -I$(CWD) -I$(CWD)/backend -I$(CWD)/includes -I$(CWD)/utils -isystem $(CWD)/modules
+BUILD_CXX_FLAGS += -DBUILD_BRIDGE -I. -I$(CWD) -I$(CWD)/backend -I$(CWD)/includes -I$(CWD)/modules -I$(CWD)/utils
 BUILD_CXX_FLAGS += -I$(CWD)/backend/engine -I$(CWD)/backend/plugin
 BUILD_CXX_FLAGS += $(LIBLO_FLAGS)
 

source/discovery/Makefile

@@ -28,7 +28,7 @@ endif
 
 # ----------------------------------------------------------------------------------------------------------------------------
 
-BUILD_CXX_FLAGS += -I$(CWD)/backend -I$(CWD)/includes -I$(CWD)/utils -isystem $(CWD)/modules
+BUILD_CXX_FLAGS += -I$(CWD)/backend -I$(CWD)/includes -I$(CWD)/modules -I$(CWD)/utils
 
 ifeq ($(MACOS),true)
 BUILD_CXX_FLAGS += -ObjC++

source/plugin/Makefile

@@ -21,7 +21,7 @@ endif
 
 # ----------------------------------------------------------------------------------------------------------------------------
 
-BUILD_CXX_FLAGS += -I$(CWD)/backend -I$(CWD)/includes -I$(CWD)/utils -isystem $(CWD)/modules
+BUILD_CXX_FLAGS += -I$(CWD)/backend -I$(CWD)/includes -I$(CWD)/modules -I$(CWD)/utils
 BUILD_CXX_FLAGS += $(NATIVE_PLUGINS_FLAGS)
 
 ifeq ($(HAVE_LIBLO),true)