Making the package detect an existing installation of spdlog

[mdorier]

It would be good if, instead of relying on a submodule, this package could detect an existing installation of spdlog. Such an installation can be detected for instance using Python's pkgconfig package, with something like the following:

try:
    import pkgconfig
    spdlog_pkgconfig = pkgconfig.parse('spdlog')
except:
    spdlog_pkgconfig = None

if spdlog_pkgconfig is None:
    spdlog_include_dirs = ['spdlog/include']
    spdlog_library_dirs = []
    spdlog_libraries    = []
    headers_to_install  = include_dir_files('spdlog/include/spdlog')
else:
    spdlog_include_dirs = spdlog_pkgconfig['include_dirs']
    spdlog_library_dirs = spdlog_pkgconfig['library_dirs']
    spdlog_libraries    = spdlog_pkgconfig['libraries']
    headers_to_install  = []

And updating the setup function as follows:

setup(
    name='spdlog',
    version='2.0.0',
    author='Gergely Bod',
    author_email='bodgergely@hotmail.com',
    description='python wrapper around C++ spdlog logging library (https://github.com/bodgergely/spdlog-python)',
    license='MIT',
    long_description='python wrapper (https://github.com/bodgergely/spdlog-python) around C++ spdlog (http://github.com/gabime/spdlog.git) logging library.',
    setup_requires=['pytest-runner'],
    install_requires=['pybind11>=2.2'],
    tests_require=['pytest'],
    ext_modules=[
        Extension(
            'spdlog',
            ['src/pyspdlog.cpp'],
            include_dirs = spdlog_include_dirs + [
                get_pybind_include(),
                get_pybind_include(user=True)
            ],
            library_dirs = spdlog_library_dirs,
            libraries = spdlog_libraries + ['stdc++'],
            extra_compile_args=["-std=c++11", "-v"],
            language='c++11'
        )
    ],
    headers=headers_to_install,
    cmdclass={'install_headers': install_headers_subdir},
    zip_safe=False,
)

Right now this is not fully functional, as it seems spdlog-python uses spdlog 1.3.1. I have tried with spdlog 1.4.1 but I get build errors so it would seem the spdlog API has changed between 1.3.1 and 1.4.1.