On my OS X El Capitan machine `which` returns 1 if it can't find the binary. This kills the bootstrap script at the crucial moment when it detects that something needs to be installed. This change ignores unsuccessful exit codes from `which`. The man page for my `which` speaks of a `-s` switch that could be used instead of the `if [ -z`, but I don't know how portable this is.
This fixes Issue 850.
* update bootstrap.sh script
+ macOS users using MacPorts are now supported (fix issue #720);
+ dependencies for macOS users using brew have been updated;
+ the boot step can now be skipped with the '-d' flag, effectively only installing dependencies
* update summary build instructions in README dependencies step
as these are meant as quick instructions, it's probably better to simply use the bootstrap.sh script available in the repo root folder, using the '-d' flag to just install the dependencies, using whatever package manager is available for the host system, rather than forcing the user to manually install those one by one
