diff --git a/c/Makefile b/c/Makefile
index fd0c367..b27449c 100644
--- a/c/Makefile
+++ b/c/Makefile
@@ -29,7 +29,7 @@
 # - CFLAGS: Any extra user-specified compiler flags (can be blank).
 
 # Recommended compiler flags:
-CFLAGS += -std=c99 -O
+CFLAGS ?= -std=c99 -O
 
 # Extra flags for diagnostics:
 # CFLAGS += -g -Wall -Wextra -Wpedantic -Wconversion -Wsign-conversion -fsanitize=undefined,address
diff --git a/cpp/Makefile b/cpp/Makefile
index f83c512..666f976 100644
--- a/cpp/Makefile
+++ b/cpp/Makefile
@@ -29,7 +29,7 @@
 # - CXXFLAGS: Any extra user-specified compiler flags (can be blank).
 
 # Recommended compiler flags:
-CXXFLAGS += -std=c++11 -O
+CXXFLAGS ?= -std=c++11 -O
 
 # Extra flags for diagnostics:
 # CXXFLAGS += -g -Wall -Wextra -Wpedantic -Wconversion -Wsign-conversion -fsanitize=undefined,address