redis: build fix and minor improvement

redis: build fix and minor improvement

The redis-001-uclibc.patch was properly adding conditionals to ensure
HAVE_BACKTRACE is not defined when uClibc is used. However, __unix
doesn't seem to be define with the toolchain Peter uses on gcc14, so
the <features.h> doesn't get included, therefore __UCLIBC isn't
defined, and the code believes that backtrace support is available. We
fix this by including <features.h> when __linux__ is defined.

Fixes:
http://autobuild.buildroot.org/results/4ba818f877af145a547a1fede0bb738192c2f181/build-end.log.

Also add another patch that do not redefine the _LARGEFILE_SOURCE and
_FILE_OFFSET_BITS macros if they are already defined on the command
line. It avoids the ugly warnings that we could see from the build
logs, and therefore make it easier to spot real errors.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>