Support building w/o Ninja for cross-compile targets

[ericgur]

I need to build a Python Extension for an OS that requires compilation with Xcode (cross compile, build on MacOS).
cmake-build-extension hard-codes the use of Ninja.
No issue having Ninja installed but it can't be used to build the project.
w/o the patch, cmake is called with 2 -G flags, one for Ninja and one for XCode.

I suggest the patch below. Check if the user (in setup.py) has asked for build system.
File modified is build_extension.py, line 149.

Maybe it can be improved because it behaves differently if Ninja is explicitly requested.

        ext_dir = Path(self.get_ext_fullpath(ext.name)).parent.absolute()
        cmake_install_prefix = ext_dir / ext.install_prefix

        ##### patch start here
        # CMake configure arguments
        configure_args = [
            f"-DCMAKE_BUILD_TYPE={ext.cmake_build_type}",
            f"-DCMAKE_INSTALL_PREFIX:PATH={cmake_install_prefix}",
        ]

        # check the user has opted-in for a build system
        build_system_specified = bool([option for option in ext.cmake_configure_options if option.startswith('**-DCMAKE_GENERATOR=**')])

        # Default to Ninja if the user didn't explicitly request a build system
        if not build_system_specified:
            configure_args +=  [
            "-GNinja",
            # Fix #26: https://github.com/diegoferigo/cmake-build-extension/issues/26
            f"-DCMAKE_MAKE_PROGRAM={shutil.which('ninja')}",
        ]

[ericgur]

Added patch file based on master/HEAD.
issue#52.patch