All GCC flags: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
Note: -Wpedantic is not included, because it always checks ISO C compatibility,
even if -std=gnuXX is given (for example, it warns about stuff that is in POSIX
but not in ISO C)
-Os = -O2 + size optimizations
-D_FORTIFY_SOURCE enables some bounds checking
-ftrapv kills the program on overflow, but is really slow
-fwrapv makes overflow always wrap (no UB)
-fanalyzer runs GCC's static analyzer
_FORTIFY_SOURCE=3 is available since glibc 2.35 and GCC 12 or LLVM 9
CPPFLAGS = C preprocessor flags
CFLAGS = C flags
CPPFLAGS:=-D_FORTIFY_SOURCE=3 -Wmissing-include-dirs -Winvalid-utf8 -finput-charset=UTF-8
CFLAGS:= -fstack-protector-strong \
-std=gnu11 -Wall -Wextra -Wconversion -Wmissing-prototypes \
-Wstrict-prototypes -Wunused-parameter -Wuninitialized -Wshadow \
-Wbad-function-cast -Wcast-qual -Wdouble-promotion -Wformat=2 \
-Wformat-overflow=2 -Wformat-signedness -Wformat-truncation=2 \
-Wnull-dereference -Winit-self -Wswitch-default \
-Wstrict-overflow=4 -Warith-conversion -Wduplicated-branches -Wduplicated-cond \
-Wlogical-op -Wold-style-definition -Wvla -Wwrite-strings -Wxor-used-as-pow \
-Wunused-macros -Wundef -Wpointer-arith -Wfloat-equal -Walloc-zero -Wunused \
-Werror -Wno-error=unused -Wno-error=unused-macros -Wno-error=float-equal \
-Wno-error=unused-but-set-variable \
-funsigned-char -fwrapv -fanalyzer \
-Os