Improve detection of X13 binary

— On GNU/Linux, the locally installed binary now has priority over the
  system-wide binary
— Do not require 32-bit binary if we are on a 64-bit system and the 64-bit
  binary is there (or vice versa)
— Code factorization in the detection logic
3 jobs for master in 1 minute and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #7147
test_matlab

00:00:46

passed #7149
test_octave

00:01:05

passed #7148
test_old_matlab

00:00:55