macOS library install_name has a minor version

[nieder]

Describe the bug On macOS, the install_name for the library is (currently) libflint.21.0.dylib, as set in configure.ac from #1342

https://github.com/flintlib/flint/blob/88eb14c722cc8771971fcf3da96efa1309745fd1/configure.ac#L900-L906

From Makefile:

FLINT_LIB:=libflint.dylib
FLINT_LIB_MAJOR:=libflint.21.dylib
FLINT_LIB_FULL:=libflint.21.0.dylib

$ otool -L libflint.dylib 
libflint.dylib:
	/Users/nieder/src/flint-3.3.1/libflint.21.0.dylib (compatibility version 21.0.0, current version 21.0.0)
	/opt/sw/lib/libmpfr.6.dylib (compatibility version 9.0.0, current version 9.2.0)
	/opt/sw/lib/gmp5/libgmp.10.dylib (compatibility version 15.0.0, current version 15.1.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1319.100.3)

Expected behavior Most libraries only use the major library version number in the install_name, signifying that the the API is stable through major library releases. Since the install_name of libflint has a minor version (21.0), does this mean that the API is not considered stable (at least to minor versions)?

System (please complete the following information):

• System CPU: macOS x86_64
• Version of FLINT (if using Git, please specify commit): 3.3.1
• How FLINT was configured (i.e. options pushed): --with-gmp --with-mpfr --with-ntl

Additional context Add any other context about the problem here.