Remove now-useless btree_gist--1.2.sql script.

Mark GiST inet_ops as opcdefault, and deal with ensuing fallout.

Create btree_gist v1.9, in which inet/cidr opclasses aren't default.

Provide more-specific error details/hints for function lookup failures.

Silence -Wmissing-variable-declarations in headerscheck.

headerscheck: Document that --with-llvm is required

headerscheck: Use ICU_CFLAGS

headerscheck: Ignore Windows-specific header