Support Windows platform

[hcho3]

• Use _putenv_s() to set environment variables, since setenv() is only available on Linux.
• Do not set FLAGS_gtest_death_test_style variable in unit tests, since doing so causes compilation failure on Windows.
• The __declspec(dllexport) specifier needs to attach every single symbol.
• Move msvc_use_static_runtime() after project() directive.
• Use Google Test 1.8.1, as 1.8.0 uses std::tr1. (MSVC 2019 dropped support for TR1.)
• Specify std::ios::binary when opening the *.params file (DL model only). This is because Windows makes distinction between "text files" and "binary files." (Unix does not make this distinction.) Opening the params file as text results in an undefined behavior.

TODO: #104

[zhiics]

LGTM