debian: drop special case CFLAGS for Alpha and PowerMac architectures

debian: drop special case CFLAGS for Alpha and PowerMac architectures

Defining HAVE_NETINET_IN_H for Alpha and __NO_STRING_INLINES for the
PowerMac QUICK bootloader date back to over two decades, to 1997 and
1998, respectively.  These two architectures are no longer supported
by Debian, and it's not clear they are actually needed in 2019 even
for someone building for these architectures.  So let's drop them and
see if anyone complains (or notices).

Signed-off-by: Theodore Ts'o <tytso@mit.edu>