glib.recipe: Use python name, not path & README.md: Recommend using QMAKE over QT5_PREFIX

README.md: Recommend using QMAKE over QT5_PREFIX

We do this internally already, but nudge users towards it too.
QT5_PREFIX is completely unusable when building with custom Qt5
builds (i.e., anything not shipped from upstream)

commit 74537c8e:

glib.recipe: Use python name not full path in tools
We don't want to hard-code the full path to the Python interpreter
in the binary output of Cerbero because it might be different on the
target system. Before:

 #!/usr/bin/env /Library/Frameworks/Python.framework/Versions/3.6/bin/python3

Which could be /usr/local/bin/python3, for instance. After:

 #!/usr/bin/env python3