Fix for building with Visual Studio

[SRombauts]

I know that two other pull request also handle this issue, but I believe that this is the smallest/cleanest way to handle the portability.

Correct the following link errors: markdown.obj : error LNK2019: unresolved external symbol _strncasecmp referenced in function _find_block_tag autolink.obj : error LNK2001: unresolved external symbol _strncasecmp html_smartypants.obj : error LNK2019: unresolved external symbol _snprintf referenced in function _smartypants_quotes

I needed to add 1 define in 3 files, betweend #if defined(_WIN32) guards :

• define strncasecmp _strnicmp

• define snprintf _snprintf

Thank you, Srombauts

[vmg]

Thank you so much. I didn't quite like the idea of adding a Windows-only header.

[SRombauts]

Thanks, this way you've mostly fixed @FSX/misaka and thus @Anomareh/mynt under Windows :)

[chobie]

Thank you, this would be helpful for PECL Sundown under Windows!