diff --git a/Source/Makefile b/Source/Makefile
index 459ec42..52fc8c9 100644
--- a/Source/Makefile
+++ b/Source/Makefile
@@ -92,21 +92,21 @@ HEADERS = \
 
 BINARY = $(APP)-$(VEC)
 
 OBJECTS = $(SOURCES:.cpp=-$(BINARY).o)
 
 EXTERNAL_OBJECTS = $(EXTERNAL_SOURCES:.h=-$(BINARY).o)
 
 # ==================================================
 # CXXFLAGS setup (and input validation)
 
-CXXFLAGS = -std=c++14 -fvisibility=hidden \
+CXXFLAGS += -std=c++14 -fvisibility=hidden \
            -Wall -Wextra -Wpedantic -Werror -Werror=shadow -Wdouble-promotion
 
 # Validate that the APP parameter is a supported value, and patch CXXFLAGS
 ifeq ($(APP),astcenc)
 # Nothing to set up
 else
 ifeq ($(APP),astcdec)
 CXXFLAGS += -DASTCENC_DECOMPRESS_ONLY
 else
 $(error Unsupported app, use APP=astcenc/astcdec)