Makefile: Switch to gnu11 from c11 due to HAL

The Nordic HAL now uses variadic macros, but in a not strictly
conformant way to the C standard, which results in thousands
of warnings when building with -Wpedantic.

Let's switch standard version to avoid this warnings.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

Makefile

@@ -31,9 +31,9 @@ SO_LIBS:=
 DEBUG:=-g
 OPT:=
 ARCH:=-m32
-WARNINGS:=-Wall -pedantic
+WARNINGS:=-Wall -Wpedantic
 COVERAGE:=
-CFLAGS:=${ARCH} ${DEBUG} ${OPT} ${WARNINGS} -MMD -MP -std=c11 \
+CFLAGS:=${ARCH} ${DEBUG} ${OPT} ${WARNINGS} -MMD -MP -std=gnu11 \
         ${INCLUDES} -fdata-sections -ffunction-sections \
         -DNRF52833_XXAA -D_XOPEN_SOURCE=500
 LDFLAGS:=${ARCH} ${COVERAGE}