Build: configure: add --with-concurrent-fencing-default option

Previously, users could change the default for the concurrent-fencing option by
defining the DEFAULT_CONCURRENT_FENCING_TRUE compile-time constant. Now, that
can be set by building with a new --with-concurrent-fencing-default=true configure
option.

When building RPMs, use the new option for RHEL >= 7 to follow what RHEL
downstream does.

Pacemaker has a number of compile-time constants used to select alternate code.
These tend to fall into the following categories:

• Implementations that depend on the build environment: HAVE_* constants, available compiler features, etc. These are auto-detected.

• Conditional building of experimental code that isn't production-ready: BUILD_PUBLIC_PACEMAKER, ENABLE_VERSIONED_ATTRS, etc. These should stay as compile-time constants.

• Conditional building of features, defaults, etc. These should be configure options, to document and expose them better to users, and allow them to show up in the features list.