`make build` fails

[simon-ess]

If you do the following:

• Install EPICS base (7.0 branch)
• Try to run make build in asyn after configuring it to point at EPICS base

then you will have a failed build failing with

make[1]: *** No rule to make target `devAsynOctet.dbd', needed by `/devEpics.dbd'.  Stop.
make: *** [asyn.build] Error 2

The reason appears to be the following code from asyn/Makefile (https://github.com/epics-modules/asyn/blob/master/asyn/Makefile#L340):

ifdef DBDCAT_COMMAND
  # Override the default command when generating devEpics.dbd
  $(COMMON_DIR)/devEpics.dbd: DBDCAT_COMMAND = \
      $(PERL) $(TOOLS)/makeIncludeDbd.pl $(devEpics_DBD) $(notdir $@)
else
  # 3.14 didn't have the DBDCAT rule:
  build: $(COMMON_DIR)/devEpics.dbd
  $(COMMON_DIR)/devEpics.dbd: $(devEpics_DBD)
    $(ECHO) "Creating dbd file $(notdir $@)"
    @$(RM) $(notdir $@)
    $(PERL) $(TOOLS)/makeIncludeDbd.pl $(devEpics_DBD) $(notdir $@)
    @$(MV) $(notdir $@) $@
endif

From what I can see, the issue is that on the first pass through in EPICS base (https://github.com/epics-base/epics-base/blob/7.0/configure/RULES) T_A is not defined which means we do not define DBDCAT_COMMAND (from RULES.Db), which adds the dependency

build: $(COMMON_DIR)/devEpics.dbd

However, on this pass COMMON_DIR is also not yet defined, so we are saying that build depends on /devEpics.dbd, which of course there is no rule to create.

A few comments:

• I'm not sure this is an asyn issue per se; this is definitely about the interaction between asyn's (custom) build rules and EPICS base.
• The build instructions for asyn explicitly say that you should run make and not make build, so technically I am not following the instructions. However, for EPICS base make and make build do the same thing; moreover, make build is not an unreasonable command to try to run, and it is surprising that it fails while make or even make install succeed.