configure: fix --with-diet-libc

configure: fix --with-diet-libc

Newer versions of autoconf pull in AC_PROG_GCC as part of
AC_CANONICAL_HOST.  So we need check for WITH_DIET_LIBC earlier in
configure.in.

Also, e2fsprogs now needs functions which are found in diet libc's
compat library.  So add support for autoconf's LIBS function, and
automatically set libs to include -lcompat.

Finally, disable compiling e4defrag by deault if --with-diet-libc is
specified because the program has too many glibc dependencies.

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