configure: Determine if musl is used for build

configure: Determine if musl is used for build

Error compiling with musl-libc:
The commit hash 810f8568f44f5863c2350a39f4f5c8d60f762958
introduces the netinet/ether.h header into xtables.h, which causes an error due
to the redefinition of the ethhdr struct, defined in linux/if_ether.h and
netinet/ether.h. This is fixed by the inclusion of -D__UAPI_DEF_ETHHDR=0 in
CFLAGS for musl. Automatically check for this macro, since it is defined
in musl but not in glibc.

Signed-off-by: Joshua Lant joshualant@gmail.com
Signed-off-by: Phil Sutter <phil@nwl.cc>