Avoid depends on bin/ folder when build install with PREFIX
Avoid depends on bin/ folder when build install with PREFIX

Fix the error if PREFIX location does not have a bin/ folder:

azureuser@LinuxBox:~/ntttcp-for-linux/src$ make install PREFIX=/home/azureuser
cp ntttcp /home/azureuser/bin/
cp: cannot create regular file '/home/azureuser/bin/': Not a directory
make: *** [Makefile:48: install] Error 1
Microsoft/ntttcp-for-linux,Microsoft/ntttcp-for-linux