Minor internal fixes, and build and portability improvements.

Changelog

[0d08be9] travis: add gcc-8 to CI (#2889) (@ahorek)
[21d2d16] [build] enable concurrent compilation (#2888) (@ahorek)
[04f59a5] Remove the link to ast_factory.hpp from Visual C++ project files (@Taritsyn)
[5ebc5d5] [build] add support of Visual Studio 2017 and 2019 to the Visual C++ project file (#2891) (@Taritsyn)
[1edc51c] Makefile: Fix STATIC_LIBSTDCPP support (@glebm)
[a46a281] Remove abspath from native Makefile (@mgreter)
[0328de2] Read files using for better portability (@glebm)
[dfe23ac] Add comparison operators for SharedImpl, fix bugs (@glebm)
[0bab610] VS2013 noexcept workaround (@glebm)