Make tinytoml compilable with Visual C++ compiler.

[peria]

Make tinytoml library compilable with Visual C++ compiler 2015.
What I did in this PR are

• Make some functions to replace with GNU extensions.
• Put some preprocessor directives to replace compiler specific attributes.
• Update CMakeLists.txt to handle compiler options correctly.
• Disable tests with multi-byte characters. Visual C++ cannot handle UTF8 without BOM.

With this PR, tinytoml can be compiled in DOS prompt.

C:\tinytoml>mkdir out
C:\tinytoml>cd out
C:\tinytoml\out>cmake ..\src -G"Visual Studio 14 2015 Win64"
C:\tinytoml\out>cmake --build .

[peria]

... and I confirmed this PR also could be built with clang+ninja on my Linux.

$ mkdir out && cd out
$ cmake ../src -GNinja
$ ninja
$ ninja test

[peria]

@mayah PTL

[mayah]

Looks OK. LGTM