[PATCH] fail gracefully when pax is not found

[Ben Gras]

https://devel.rtems.org/ticket/2437

Attachment to that ticket is a clean fix for:

If pax is not found by configure, the empty PAX variable is used in e.g.
this line:

$(PAX) -w -f $@ $<

Causing the 'w' command to be invoked:

21:37:25 up 6 min, 3 users, load average: 1,90, 1,14, 0,49

USER TTY LOGIN@ IDLE JCPU PCPU WHAT
../../../../../../tools/build/rtems-bin2c -C dl.tar dl-tar.c
../../../../../../tools/build/rtems-bin2c -H dl.tar dl-tar.h
cannot open dl.tar for reading
cannot open dl.tar for reading
make[6]: * [dl-tar.c] Error 1
make[6]: * Waiting for unfinished jobs....
make[6]: * [dl-tar.h] Error 1
make[6]: Leaving directory
`/home/beng/development/rtems/b-beagle/arm-rtems4.11/c/beagleboneblack/testsuites/libtests/dl01'
make[5]: * [all-local] Error 1
make[5]: Leaving directory
`/home/beng/development/rtems/b-beagle/arm-rtems4.11/c/beagleboneblack/testsuites/libtests'
make[4]: * [all] Error 2
make[4]: Leaving directory
`/home/beng/development/rtems/b-beagle/arm-rtems4.11/c/beagleboneblack/testsuites/libtests'
make[3]: * [all-recursive] Error 1
make[3]: Leaving directory
`/home/beng/development/rtems/b-beagle/arm-rtems4.11/c/beagleboneblack/testsuites'
make[2]: * [all-recursive] Error 1
make[2]: Leaving directory
`/home/beng/development/rtems/b-beagle/arm-rtems4.11/c/beagleboneblack'
make[1]: * [all-recursive] Error 1
make[1]: Leaving directory
`/home/beng/development/rtems/b-beagle/arm-rtems4.11/c'
make: * [all-recursive] Error 1


Thanks to several reporters of this problem, most recently Debajyoti
Majumdar and Claudio Scordino, while trying to build the tests with
the Beagle BSP.