Most importantly, differentiate between library and non-library components. The former can be provided and required, and if provided but disabled, can be replaced by pre-installed versions. The latter can only be provided and cannot be the target of a dependency relationship.
Previously, we always invoked AX_PKG_CONFIG_CHECK() for a disabled component (i.e. a component which we provide but which was disabled on the configure command line). This is both excessive (because we only need to run this check for components that will be needed during the build) and insufficient (because we also need to run it for components from other Cryb projects).
These macros are designed to work in concert with the AX_PKG_CONFIG macros. It is now possible to disable any component, even if another depends on it, provided that it is already installed. For enabled components, we define the same variables as AX_PKG_CONFIG would, so Makefiles don't need to know if a component they rely on is being built as part of this packge or if it was already installed.
