diff --git a/GNUmakefile b/GNUmakefile
index 6fee54d3..3ee9a812 100755
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -119,9 +119,10 @@ else ifeq ($(findstring distclean,$(MAKECMDGOALS)),trim)
   DETECT_FEATURES := 0
 endif
 
-# Strip out -Wall, -Wextra and friends for feature testing
+# Strip out -Wall, -Wextra and friends for feature testing. FORTIFY_SOURCE is removed
+# because it requires -O1 or higher, but we use -O0 to tame the optimizer.
 ifeq ($(DETECT_FEATURES),1)
-  TCXXFLAGS := $(filter-out -D_FORTIFY_SOURCE=1 -D_FORTIFY_SOURCE=2 -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
+  TCXXFLAGS := $(filter-out -D_FORTIFY_SOURCE=% -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
   ifneq ($(strip $(TCXXFLAGS)),)
     $(info Using testing flags: $(TCXXFLAGS))
   endif
diff --git a/GNUmakefile-cross b/GNUmakefile-cross
index d32139ed..a6585253 100755
--- a/GNUmakefile-cross
+++ b/GNUmakefile-cross
@@ -192,9 +192,10 @@ else ifeq ($(findstring distclean,$(MAKECMDGOALS)),trim)
   DETECT_FEATURES := 0
 endif
 
-# Strip out -Wall, -Wextra and friends for feature testing
+# Strip out -Wall, -Wextra and friends for feature testing. FORTIFY_SOURCE is removed
+# because it requires -O1 or higher, but we use -O0 to tame the optimizer.
 ifeq ($(DETECT_FEATURES),1)
-  TCXXFLAGS := $(filter-out -D_FORTIFY_SOURCE=1 -D_FORTIFY_SOURCE=2 -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
+  TCXXFLAGS := $(filter-out -D_FORTIFY_SOURCE=% -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
   ifneq ($(strip $(TCXXFLAGS)),)
     $(info Using testing flags: $(TCXXFLAGS))
   endif