cmocka: Add VS2015 support

Visual Studio now includes support for vsnprintf and snprintf but does
so as an inline functions. check_symbol_exists is used to check for
its existence within stdio. [3] All tests are passing when run from
within the IDE or via NMake. Command line builds via msbuild seem to
error for the first few example tests.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>