build: nacl: Propagate nacl specific environment variables to parent shell

Use source command to execute the nacl_build.sh script in order to
change parent shell's environment variable needed to find the just built
libnacl.

wilson says:

> As I use "make nacl" to install nacl, the nacl_path.sh would be execute.
> Then NACL_INC_DIR env variable will be set and appended to ~/.bashrc. In
> nacl_build.sh, it used 'source' command to execute nacl_path.sh, however
> the NACL_INC_DIR variable only took effect in this script context, but
> not in the shell I was running.
> I think we need the 'source' command to execute nacl_build.sh so that we
> don't need to restart the shell for further installation.

Signed-off-by: wilson <wilson.wen.chn@gmail.com>
[tklauser: Reformated commit message]
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>