--- Makefile.orig	2019-10-08 19:10:51 UTC
+++ Makefile
@@ -20,11 +20,16 @@ LDFLAGS_gdb := $(LDFLAGS_shared)
 
 COMPTIMEPLACE := -D'COMPILATION_TIME_PLACE="$(shell echo `date` $(HOSTNAME):`pwd`)"'
 
-CXXFLAGS_common := -pipe -std=c++11 -Wall -Wextra -fopenmp $(COMPTIMEPLACE)
-CXXFLAGS_main := -O3 $(CXXFLAGS_common)
-CXXFLAGS_gdb :=  -O0 -g $(CXXFLAGS_common)
+# Defaults, overridden by make arguments or environment
+CXXFLAGS ?= -pipe -O3 -Wall -Wextra
+CFLAGS ?= -pipe -O3 -Wall -Wextra
 
-CFLAGS := -O3 -pipe -Wall -Wextra $(CFLAGS)
+# Unconditionally set essential flags
+CXXFLAGS_common := -std=c++11 -fopenmp $(COMPTIMEPLACE)
+CXXFLAGS_main += $(CXXFLAGS_common)
+
+# -O or -O2 should work fine with gdb as well
+CXXFLAGS_gdb :=  -O0 -g $(CXXFLAGS_common)
 
 
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