Test for sys/syscall.h in configure to fix dietlibc build problem

Test for sys/syscall.h in configure to fix dietlibc build problem

When compiling with dietlibc, sys/syscall.h isn't supported; as of
dietlibc 0.30, it exists but it references a non-existent asm/unistd.h
header file.  So we have to test for its existence and avoid using it
in lib/uuid/gen_uuid.c if it is not supported.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>