Ensure PACKAGE_VERSION is set in the Makefile

[daviesrob]

This variable is set by expanding $(shell version.sh). As this is not in a normal Makefile rule, failing to run version.sh just results in an empty variable. To check that it really was set, add a check to ensure it's not empty in the rule that builds the version.h file.

See also samtools/samtools#2337. Failing here is unlikely to result in broken files, but we should probably catch it anyway.

[jmarshall]

Because PACKAGE_VERSION is constructed using GNU Make-specific functions, there's probably no downside to a GNU Make-specific check. E.g., how about something like

 PACKAGE_VERSION := $(shell $(srcdir)/version.sh)
+ifeq "$(PACKAGE_VERSION)" ""
+$(error PACKAGE_VERSION is not defined.  Check version.sh works)
+endif

Which admittedly differs from your version in not letting you avoid the check by manually making version.h. Which is either a good or a bad thing.

[daviesrob]

True, but I preferred a more traditional solution, although it did get a bit more complicated by hiding the if statement from the make output. I did also ponder running version.sh in the make rule so its failure would be detected automatically, but I think the bit that quietly detects if version.h needs a rebuild is useful.