diff --git a/Makefile.gnu b/Makefile.gnu
index 92f6358..293ee97 100644
--- a/Makefile.gnu
+++ b/Makefile.gnu
@@ -15,7 +15,8 @@ LIBRARIES = -lstdc++
 
 MODULES = $(SRCS:.c=.o)
 MODULES := $(MODULES:.cpp=.o)
-CFLAGS ?= -O3 -fPIC -fexceptions -fvisibility=hidden
+CFLAGS ?= -O3
+CFLAGS += -fPIC -fexceptions -fvisibility=hidden
 # OpenJPEG
 CFLAGS += -DOPJ_STATIC
 # LibRaw
@@ -23,7 +24,8 @@ CFLAGS += -DNO_LCMS
 # LibJXR
 CFLAGS += -DDISABLE_PERF_MEASUREMENT -D__ANSI__
 CFLAGS += $(INCLUDE)
-CXXFLAGS ?= -O3 -fPIC -fexceptions -fvisibility=hidden -Wno-ctor-dtor-privacy
+CXXFLAGS ?= -O3
+CXXFLAGS += -fPIC -fexceptions -fvisibility=hidden -Wno-ctor-dtor-privacy
 # LibJXR
 CXXFLAGS += -D__ANSI__
 CXXFLAGS += $(INCLUDE)