eal/windows: fix debug build with MinGW

eal/windows: fix debug build with MinGW

Compiling with MinGW in --buildtype=debug produces a redefinition
error for strncasecmp.

The root cause is that rte_os.h shouldn't be injecting POSIX definitions
into the environment.  It is the applications responsibility to decide
how to handle missing functionality.

Resolving this properly will require further work, but in the meantime
wrap all such definitions with #ifndef/#endif.  This resolves the specific
issue with strncasecmp and handles similar issues that applications may
encounter.

Fixes: e8428a9d89f1 ("eal/windows: add some basic functions and macros")
Cc: stable@dpdk.org
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Nick Connolly <nick.connolly@mayadata.io>