#4265 CMake generator IS optional on Windows

[uilianries]

Hi!

• When arch is not declared and only os_build is available,
  both Linux and Macos can run CMake to generate a default
  build tool e.g. Makefiles. However, on Windows this is not
  true and raises an error.

• This commit returns "MinGW Makefiles" for Windows when arch
  is not specified.

• I've added unit tests to check on Windows, Linux and Macos.

Changelog: Fix: Fixes default CMake generator on Windows to use MinGW Makefiles.
Docs: conan-io/docs#1026
closes #4265

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[SSE4]

why MinGW Makefiles is default choice? (e.g. not NMake makefiles or Unix makefiles)
as I understand, it requires MinGW make, which is not something always available, right?

[uilianries]

That's true, but if think about, both OSX Linux and MacOS have default outputs.

Also, I agree with NMake and Unix Makefiles, but our most common case on Windows is MinGW Makefiles when MSVC is not listed.

[SSE4]

okay, need to fix the following and we're good:

======================================================================

FAIL: test_missing_settings (blocked_v2.conans.test.unittests.client.build.cmake_test.CMakeTest)

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/tmp/source/341f/py27/blocked_v2/conans/test/unittests/client/build/cmake_test.py", line 1040, in test_missing_settings

    self.assertEquals(cmake.generator, None)

AssertionError: 'MinGW Makefiles' != None

[lasote]

Hi @uilianries! The Changelog: (Fix): Fixes #4265 is not valid, we use that line to build the changelog. So please add a description there (without the brackets in the Fix word) and move the "Fixes #..." to another line.
Docs are also required, probably it deserves to be mentioned in the CMake build helper reference. When docs are not needed you would have to specify "Docs: omit", that is also taken into account for the changelog.

[uilianries]

@lasote Done

I didn't find any information about default generators on Docs, so I omitted.

[lasote]

I think we could add some words at https://docs.conan.io/en/latest/reference/build_helpers/cmake.html in the generators parameter. Explaining what is the detected generator depending on the settings etc.

[uilianries]

Agreed! I'll do it soon.

[memsharded]

client.out also works

In theory these checks for error are not necessary, as client.run() will raise in this cases.