added mingw test platform (#829)

* Provide a minimal reimplementation of our autoconf, to try windows builds

* Try building with windows

* Fix thinko in spelling

* Ensure shell script runs inside a shell

* Add a hack to aid include discovery

* Just keep adding tech debt...

* Assume that we will have slashes in some of the replacement strings and avoid that char with sed

* Restore one slash

* Hack around the tools makefile interdependancy bug

* A correct cflags include hack for each compile dir

* Ensure we link against winsock (note, even though this says 32bit, it should link the 64bit library ... I think)

* Bad link ordering if we dont use LDLIBS

* Remove unused make variable

* Remove makefile duplication using inheritance (this does mean you can no longer cd tools; make, but must do make tools)

* Add missing library for win32

* Show OS variable

* Make hack autoconf more robust for tests on non gitlab runners

* Remove no longer used substitutions from hack autoconf

* Add missing include path to tools under win32

* Build the win32 subdir when the compiler is Msys

* The different subdirs have different dependancies

* Ensure we can find the include files

* Fix library link ordering

* Ensure the tools dir can find the special win32 lib

* Deal with the differing basic type sizes on both linux/64bit and windows/64bit

* Document the steps to mimic the github windows/mingw build locally - to allow for simpler debugging

* Ensure branch name in instructions matches my test branch name

* Clarify the shell needed to build with mingw

* Since the makefile depends on knowing the OS, raise a fatal error if we cannot determine this

* Handling different compile environments is hard.

- Linux: sane and reasonable results for both uname -s (=Linux) and
  uname -o (=GNU/Linux)
- Windows/Mingw: insane results for uname -s
  (=MSYS_NT-$MAJOR.$MINOR-$BUILDNR) but sane results for uname -o (Msys)
- Macos: sane results for uname -s (=Darwin) but does not support
  uname -o at all

* Revamp the way that Mingw is detected

* Avoid attempting to generate gcovr report when running under windows

* Whoops, isolate the right step

* Fix spelling mistake

* win32/Makefile: Remove unused setting and add comment

* ensure that all win32 includes use the same expected path

* Allow simpler cross compilation by letting configure pass the CC and AR environment through

* Avoid multiple '_CRT_SECURE_NO_WARNINGS redefined' warnings

* Convert to a consolidated CONFIG_TARGET variable to select any different compile options

* Use the more generic printf defines to avoid warnings on mingw

* Update mingw build docs

* English better for reader happy make

* Address a number of mingw compiler warnings

* Fix Visual C compile

* Be sure to document some of the hacky nature of the mingw build

doc/Building.md

@@ -12,7 +12,7 @@ If you are on a modern version of macOS (i.e. Catalina), the commands above will
 For more information refer to vendor documentation or the [Apple Technical Note](https://developer.apple.com/library/content/technotes/tn2459/_index.html).
 
 
-# Build on Windows
+# Build on Windows (Visual Studio)
 
 ## Requirements
 
@@ -46,8 +46,9 @@ In order to run n2n, you will need the following:
 - If OpenSSL has been linked dynamically, the corresponding `.dll` file should be available
   onto the target computer.
   
-NOTE: Sticking to this tool chain ensures that resulting executables are able to communicate with Linux or other OS builds.
-Especialy MinGW builds are reported to not be compatible to other OS builds, please see [#617](https://github.com/ntop/n2n/issues/617) and [#642](https://github.com/ntop/n2n/issues/642).
+NOTE: Sticking to this tool chain has historically meant that resulting
+executables are more likely to be able to communicate with Linux or other
+OS builds, however efforts are being made to address this concern.
 
 ## Build (CLI)
 
@@ -97,6 +98,29 @@ Here is an example `supernode.conf` file:
 
 See `edge.exe --help` and `supernode.exe --help` for a full list of supported options.
 
+# Build on Windows (MinGW)
+
+These steps were tested on a fresh install of Windows 10 Pro with all patches
+applied as of 2021-09-29.
+
+- Install Chocolatey (Following instructions on https://chocolatey.org/install)
+- from an admin cmd prompt
+    - choco install git mingw make
+- All the remaining commands must be run from inside a bash shell ("C:\Program Files\Git\usr\bin\bash.exe")
+    - git clone $THIS_REPO
+    - cd n2n
+    - ./scripts/hack_fakeautoconf
+    - make
+    - make test
+
+Due to the hack used to replace autotools on windows, any build created this
+way will currently have inaccurate build version numbers.
+
+Note: MinGW builds have a history of incompatibility reports with other OS
+builds, please see [#617](https://github.com/ntop/n2n/issues/617) and [#642](https://github.com/ntop/n2n/issues/642).
+However, if the tests pass, you should have a high confidence that your build
+will be compatible.
+
 # General Building Options
 
 ## Compiler Optimizations