Updating extlib to 1.7.7 and build with minimal=1

[Andy Li <andy@onthewings.net>]

Hi,

I've just pushed some commits to extlib's salsa repo.

I changed d/rules to build it with minimal=1, which is also used by
the OPAM extlib package. The flag excludes Unzip, UChar, and UTF8,
which conflict with other OCaml code including the Haxe 4.1.0 code
base, which I was about to package.

I checked the reverse dependencies of extlib with `apt-rdepends
--reverse libextlib-ocaml-dev`:
  * libcudf-ocaml-dev (0.7-5+b2). It doesn't use Unzip, UChar, and
UTF8 as it build-depends on either "extlib" (minimal=1) and
"extlib-compat" (full) according to its OPAM package.
  * ocaml-libs (4.08.1.2). Just a meta package.

It would be great if anyone (Stéphane?) can review and upload. I'm not
sure if I've missed anything.

Best regards,
Andy