If the strings match, git__fnmatch returns GIT_SUCCESS and
GIT_ENOMATCH on failure to match.

MSVC fixes: Added a test for _MSC_VER and (in that case) defined
HAVE_STRING_H to 1 so it doesn't try to include <strings.h> which
doesn't exist in the MSVC world. Moved the function declarations to
use the modern inline ones so MSVC doesn't have a fit. Added casts
everywhere so MSVC doesn't crap its pants.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>