I tested make on FreeBSD 9.1R.
BSD Make fails `$(add-prefix ...)` and `$(shell ...)`, it seems to take much more work to handle both BSD and GNU Make.

For example, following works on GNU's and doesn't work on BSD's
```
PLATFORM = $(shell $(PYTHON) -c "import sys; print(sys.platform)")
```
works on BSD's and doesn't work on GNU's
```
PLATFORM != $(PYTHON) -c "import sys; print(sys.platform)"
```