Check for pip in prereqs.py

[msoeken]

Is your feature request related to a problem? Please describe.
If pip is not installed, but Python is, prereqs.py may exit successfully, but build.py will fail with a Python exception. Especially in virtual environments, pip may not necessarily be installed initially.

Describe the solution you'd like
Check for pip in prereqs.py

Describe alternatives you've considered
Do nothing and notice that pip is installed once build.py is called the first time.