autofs: really fix build

The autofs configure script does a number of weird things.

First, instead of using the default search path for AC_CHECK_PROGS(),
it overrides it its own search path, ignoring the PATH environment
variable. Due to this, autofs fails to find flex even though flex is
available in the PATH. Fortunately, the authors provided a --with-path
configure option which allows to override this search path.

In addition, the configure script is checking on the build machine
that a number of programs are available, while those programs are in
fact only needed on the target. The one causing build issues is
modprobe, because it may not necessarily be installed on build
machines, so we explicitly tell autofs that modprobe is located in
/sbin/modprobe. It might be necessary to provide additional values for
other programs, as we discover other problems.

Fixes:

  http://autobuild.buildroot.org/results/447/4479382b2d4d762991cab2e93fd08ad6160c2921/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>