Fix C builds (#8795)

Missing stdbool.h for 'bool' in features .h, at least one user is arduinoWebSockets
Adds minimal headers sanity-check script to verify that C builds work as expected

Also noticed and removed default argument from crc32() in internal .h that may be used in .c
(not sure how extern C & default worked simultaniously, but at least in our .cpp Gcc somehow figured out it is a no overload solution)

tests/sanity_check.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+
+root=$(git rev-parse --show-toplevel)
+source "$root/tests/common.sh"
+
+pushd "$root"/tools
+python3 get.py -q
+
+popd
+pushd "$cache_dir"
+
+gcc="$root/tools/xtensa-lx106-elf/bin/xtensa-lx106-elf-gcc"\
+" -I$root/cores/esp8266"\
+" -I$root/tools/sdk/include"\
+" -I$root/variants/generic"\
+" -I$root/tools/sdk/libc/xtensa-lx106-elf"
+
+$gcc --verbose
+
+set -v -x
+
+cat << EOF > arduino.c
+#include <Arduino.h>
+EOF
+
+$gcc -c arduino.c
+
+cat << EOF > coredecls.c
+#include <coredecls.h>
+EOF
+
+$gcc -c coredecls.c
+
+cat << EOF > features.c
+#include <core_esp8266_features.h>
+EOF
+
+$gcc -c features.c
+
+cat << EOF > sdk.c
+#include <version.h>
+EOF
+
+$gcc -c sdk.c