Use POSIX standard technique for testing existance of a command

Fixes issue Kitware#73.

macOS dirname does not have a `--version` option, which was making the script fail on macOS.

This technique should be POSIX compliant and more crossplatform.

Solution taken from:

https://stackoverflow.com/questions/592620/how-can-i-check-if-a-program-exists-from-a-bash-script

Utilities/updateBoost.sh

@@ -24,9 +24,8 @@ die()
 ## Validate ##
 required_commands=( git grep dirname basename cat )
 for required_command in ${required_commands[@]}; do
- $required_command --version >/dev/null 2>&1
- if [[ $? -ne 0 ]]; then
- die "Command \"$required_command\" not found"
+ if ! command -v $required_command &> /dev/null; then
+ die "Command \"$required_command\" not found"
  fi
 done