Provide a second optional argument to PKG_PROG_PKG_CONFIG with an action to
take if no pkgconf is found, and default to aborting.
Too many configure scripts in the wild don't handle an empty PKG_CONFIG,
resulting in weird error messages through the run, which often confuse
users.
Authors wishing to fallback to other methods can either return to the old
behavior by specifying [:] as an action, or use [PKG_CONFIG=false] and call
pkg-config macros normally, handling the fallback as any other failure.
Users can override PKG_CONFIG in the command line so this should imply no
regression.
Remove the 'traverse_serial' fields which were added in 2.1.1.
Use the 'serial' field to track the current traversal.
Stop using 'identifier' to sort packages in the flattened solution.
Directly construct the flattened solution by a specific walk which
also preserves the relative order in Requires and Requires.private.
The topological sort is a single list, so don't fill requires_private.
Purely private dependencies are marked in dependency flags.
The ancestor flag is a pkg property, not a client property.
