optional.hpp: use #if defined(_MSC_VER) to avoid -Wundef warning (#65, thanks @eepp)

Signed-off-by: Philippe Proulx <[email protected]>

Author: eepp

include/nonstd/optional.hpp

@@ -47,7 +47,7 @@
 // Control presence of exception handling (try and auto discover):
 
 #ifndef optional_CONFIG_NO_EXCEPTIONS
-# if _MSC_VER
+# if defined(_MSC_VER)
 # include <cstddef>     // for _HAS_EXCEPTIONS
 # endif
 # if defined(__cpp_exceptions) || defined(__EXCEPTIONS) || (_HAS_EXCEPTIONS)