make control function "error" is nice for users but breaks introspection of dependencies

[klauer]

Lines such as this that employ the "control function" error defined by GNU make:
https://github.com/pcdshub/ads-ioc/blob/b2cb4a015cb062dc2be62a38f6418c7551e92f76/configure/RELEASE#L47

Are useful for debugging when trying to build ads-ioc and having a missing dependency.
However, it breaks introspection of dependencies via tools such as whatrecord deps (or epics-sumo, presumably) if dependencies are missing.

I think it may be useful to introduce a convention wherein:

• DEPENDENCY_CHECKER gets set to 1 by tools like whatrecord (à la typing.TYPE_CHECKING)
• If unset (DEPENDENCY_CHECKER=0), $(error) get called as usual
• If set (DEPENDENCY_CHECKER=1), either no output or some warning text could take place of $(error):
  • Warnings could be picked up by the tool or dropped

This could look like:

_DEPENDENCY_CHECKER ?= 0

ifneq ($(_DEPENDENCY_CHECKER),1)

# Check for valid EPICS_BASE
ifeq ($(wildcard $(EPICS_BASE)/include),)
$(error Invalid EPICS_BASE: $(EPICS_BASE)/include)
endif

endif

[klauer]

Wrapped this into #86 - closing