lsof: don't use host toolchain to determine target libc

The lsof Configure script builds a test program using the host toolchain to
test whether glibc is used. This test is broken in cross compilation
environment. Set LINUX_CLIB to avoid the test. This should give the correct
result even for non glibc toolchains, since all libc variants we support have
the netinet/tcp.h header.

Fixes:
http://autobuild.buildroot.net/results/a1f/a1f0572dbf968c21f70b35cefff7ef7a1d9a348a/



Cc: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>