This patch implements snprintf_l function in a way similar to the other
functions in src/support/win32/locale_win32.cpp and locale_win32.h,
calling upon vsnprintf for which there is a MingW correct alternative.

Note! __USE_MINGW_ANSI_STDIO is not modified in this patch. In order to
use the __mingw version it must be defined before including the MingW
headers.