Build: spec: defines instead of some globals + error suppression

...together with cosmetic cleanup at the affected places.

Point of using %define rather than %global is that these are expanded
lazily, meaning that shell executions ("%()") can be deferred if executed
at all. Also make such executions silent for pre-build cases of parsing
the spec, e.g., to get the list of build requirements ("rpmbuild -bs"
using a freshly installed srpm in a minimalistic build environment)
otherwise messages like these can be emitted:

Package corosync was not found in the pkg-config search path.
Perhaps you should add the directory containing `corosync.pc'
to the PKG_CONFIG_PATH environment variable
No package 'corosync' found
sh: /usr/bin/python: No such file or directory

This is currently the case in non-customized mock-based (here COPR)
builds.