Hi,

I've been experimenting with musl-libc Gentoo systems. I used the HEAD
of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git

I found that btrfs-progs does not compile with musl-libc, for a few reasons:

* It makes use of the private glibc __always_inline macro.

* Various headers that should be included are not included:
linux/limits.h and limits.h. (for XATTR_SIZE_MAX and PATH_MAX)

* backtrace() using execinfo.h is enabled by default; execinfo.h is
glibc-specific and thus does not work on other libc's. musl does not
support it, and I think uclibc also does not support it.

I have worked around the problems in the following way:

* Define __always_inline if __glibc__ is not defined. This is arguably
the most clean solution. It would be better to simply not use the
__always_inline macro (instead, use __attribute__) throughout
btrfs-progs, but I was not sure what the developers would prefer. This
is currently done in kerncompat.h, but you may want to move that to
another file.

* Include various headers where required.

* If __glibc__ is not defined, define BTRFS_DISABLE_BACKTRACE. Currently
the define magic happens in kerncompat, because that also where BTRFS
includes execinfo. Personally, I think it would make more sense to
always disable backtrace instead of enabling it by default -- but
perhaps in a testing phase, enabling it by default in the sensible choice.

Attached are the two patches generated with git format-patch. I am aware
that this may not be required format for submitting patches -- but
please give me some time to get used to the etiquette. :-)

Please let me know if musl-libc (or any other libc) is a supported
platform, and if so, if and how I can improve on said patches.

Regards,
Merlijn