Makefile: detect compiler and enable more warnings in DEVELOPER=1

The set of extra warnings we enable when DEVELOPER has to be
conservative because we can't assume any compiler version the
developer may use. Detect the compiler version so we know when it's
safe to enable -Wextra and maybe more.

These warning settings are mostly from my custom config.mak a long
time ago when I tried to enable as many warnings as possible that can
still build without showing warnings. Some of those warnings are
probably worth fixing instead of just suppressing in future.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

detect-compiler

@@ -0,0 +1,53 @@
+#!/bin/sh
+#
+# Probe the compiler for vintage, version, etc. This is used for setting
+# optional make knobs under the DEVELOPER knob.
+
+CC="$*"
+
+# we get something like (this is at least true for gcc and clang)
+#
+# FreeBSD clang version 3.4.1 (tags/RELEASE...)
+get_version_line() {
+	$CC -v 2>&1 | grep ' version '
+}
+
+get_family() {
+	get_version_line | sed 's/^\(.*\) version [0-9][^ ]* .*/\1/'
+}
+
+get_version() {
+	get_version_line | sed 's/^.* version \([0-9][^ ]*\) .*/\1/'
+}
+
+print_flags() {
+	family=$1
+	version=$(get_version | cut -f 1 -d .)
+
+	# Print a feature flag not only for the current version, but also
+	# for any prior versions we encompass. This avoids needing to do
+	# numeric comparisons in make, which are awkward.
+	while test "$version" -gt 0
+	do
+		echo $family$version
+		version=$((version - 1))
+	done
+}
+
+case "$(get_family)" in
+gcc)
+	print_flags gcc
+	;;
+clang)
+	print_flags clang
+	;;
+"FreeBSD clang")
+	print_flags clang
+	;;
+"Apple LLVM")
+	print_flags clang
+	;;
+*)
+	: unknown compiler family
+	;;
+esac