Change shared library installation naming on macOS

It is not customary to install a shared library with a minor version
number (libpq.5.16.dylib) on macOS.  We just need the file with the
major version number (libpq.5.dylib) and the one without version
number (libpq.dylib).  This also matches the installation layout used
by Meson.

Discussion: https://www.postgresql.org/message-id/e0c44fb2-8b66-a4b9-b274-7ed3a1a0ab74@enterprisedb.com

Author: petere

src/Makefile.shlib

@@ -122,7 +122,7 @@ ifeq ($(PORTNAME), darwin)
       version_link	= -compatibility_version $(SO_MAJOR_VERSION) -current_version $(SO_MAJOR_VERSION).$(SO_MINOR_VERSION)
     endif
     LINK.shared		= $(COMPILER) -dynamiclib -install_name '$(libdir)/lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)' $(version_link) $(exported_symbols_list) -multiply_defined suppress
-    shlib		= lib$(NAME).$(SO_MAJOR_VERSION).$(SO_MINOR_VERSION)$(DLSUFFIX)
+    shlib		= lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)
     shlib_major		= lib$(NAME).$(SO_MAJOR_VERSION)$(DLSUFFIX)
   else
     # loadable module