Makefile uniformity review against hybrid_lib_ada

[abitofhelp]

Context

hybrid_lib_ada is being updated with Makefile improvements harvested from astfmt. After that lands, tzif_ada should be reviewed for the same updates to maintain ecosystem uniformity.

Wait for

• hybrid_lib_ada Makefile backport (tracked separately)

Then apply

• `ALR_BUILD_FLAGS` add `-XGNATCOLL_ICONV_OPT=`
• New `ALR_RELEASE_FLAGS` (iconv + static lib type)
• `build-release` uses release flags
• `build-tests` stricter error handling
• `compress` help text fix

Acceptance

• tzif_ada Makefile diffs against hybrid_lib_ada are intentional only
• PR opened + GPT review + merge

References

• astfmt Makefile (origin of these improvements)
• hybrid_lib_ada parent change (TBD — link when filed)