misc/util.c: enable MinGW alarm() when building for Windows

misc/util.c: enable MinGW alarm() when building for Windows

To compile for Windows, this file needs MinGW's implementation of
alarm().  To expose that definition, some macros must be defined before
including the system headers.  This was done in Android.bp, but it was
not done in the autotools-based build system.  Define these macros in
the source file itself so that all build systems work.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>