package/btrfs-progs: fix build issues under musl

Fixes http://autobuild.buildroot.net/results/b97/b97f1e9cd459da96e3e1680bb7c43d8103ab12c2/build-end.log



Patches by Thomas to refactor detection of __always_inline and backtrace
detection. Also added #include <limits.h> for PATH_MAX.

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>