Fix build of internal plugins under FreeBSD

source/native-plugins/external/Makefile.mk

@@ -98,6 +98,7 @@ endif
 DPF_FLAGS  = -I$(CWDE)/modules/distrho
 
 ifeq ($(HAVE_DGL),true)
+DPF_FLAGS += $(shell pkg-config --cflags gl)
 DPF_FLAGS += -I$(CWDE)/modules/dgl -DDGL_NAMESPACE=CarlaDGL -DDGL_FILE_BROWSER_DISABLED -DDGL_NO_SHARED_RESOURCES
 endif