./configure --enable-debug generates -O2 gcc option, overriding earlier -Og option which is the correct optimization level for -g debugging.

CFLAGS = -g -O2
[...]
CXXFLAGS = -Og -g3 -Wstack-protector -fstack-protector-all -Wall -Wextra -Wformat -Wvla -Wformat-security -Wno-unused-parameter -g -O2

Ubuntu 16 and OSX 10.13