sanei_pp: fix [-Wcpp] compiler warning

The `#warning` directive is turned into an error by `-Werror`.  The
directive triggers is guaranteed to trigger with __GNUC__ when no
parallel support is available.  This would prevent strict CI builds.

Output a message with all compilers that understand the pragma.  Those
that don't should ignore it, according to C99.

sanei/sanei_pp.c

@@ -126,9 +126,7 @@ inb( u_long port )
 #elif defined(HAVE_LIBIEEE1284)
 # include <ieee1284.h>
 #else
-# if defined(__GNUC__)
-#  warning "No I/O support for this architecture!"
-# endif
+# pragma message "No I/O support for this architecture!"
 # define IO_SUPPORT_MISSING
 #endif