Run the settings generator only when the build changes

A build is considered changed if either the TARGET, CFLAGS or
the #define'd constants by including target.h and platform.h change

The Makefile now runs a new tool in src/utils/build_stamp.rb which
updates the file in obj/build.stamp iff the build has changed. This
allows us to make the generated files to depend on this stamp file
rather than rebuilding them all the time. Automatic dependencies
generated by the compiler take care of recompiling just the files
which depend on the generated ones.

src/utils/compiler.rb

@@ -0,0 +1,96 @@
+#!/usr/bin/env ruby
+
+# This file is part of INAV.
+#
+# author: Alberto Garcia Hierro <alberto@garciahierro.com>
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# Alternatively, the contents of this file may be used under the terms
+# of the GNU General Public License Version 3, as described below:
+#
+# This file is free software: you may copy, redistribute and/or modify
+# it under the terms of the GNU General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or (at your
+# option) any later version.
+#
+# This file is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+# Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see http://www.gnu.org/licenses/.
+
+require 'digest'
+require 'open3'
+require 'rbconfig'
+require 'shellwords'
+
+class Compiler
+    def initialize
+        # Look for the compiler in PATH manually, since there
+        # are some issues with the built-in search by spawn()
+        # on Windows if PATH contains spaces.
+        dirs = (ENV["PATH"] || "").split(File::PATH_SEPARATOR)
+        bin = "arm-none-eabi-g++"
+        dirs.each do |dir|
+            p = File.join(dir, bin)
+            ['', '.exe'].each do |suffix|
+                f = p + suffix
+                if File.executable?(f)
+                    if @verbose
+                        puts "Found #{bin} at #{f}"
+                    end
+                    @path = f
+                    return
+                end
+            end
+        end
+        raise "Could not find #{bin} in PATH, looked in #{dirs}"
+        @verbose = ENV["V"] == "1"
+    end
+
+    def default_args
+        cflags = Shellwords.split(ENV["CFLAGS"] || "")
+        args = [@path]
+        cflags.each do |flag|
+            # Don't generate temporary files
+            if flag == "" || flag == "-MMD" || flag == "-MP" || flag.start_with?("-save-temps")
+                next
+            end
+            if flag.start_with? "-std="
+                flag = "-std=c++11"
+            end
+            if flag.start_with? "-D'"
+                # Cleanup flag. Done by the shell when called from
+                # it but we must do it ourselves becase we're not
+                # calling the compiler via shell.
+                flag = "-D" + flag[3..-2]
+            end
+            args << flag
+        end
+        return args
+    end
+
+    def run(input, output, args = nil)
+        all_args = default_args
+        if args
+            all_args.push(*args)
+        end
+        all_args << "-o" << output << input
+        stdout, stderr = Open3.capture3(join_args(all_args))
+        return stdout, stderr
+    end
+
+    private
+
+    def join_args(args)
+        if RbConfig::CONFIG['host_os'] =~ /mswin|mingw/
+            return args.join(' ')
+        end
+        return Shellwords.join(args)
+    end
+end