Fix incorrect precedence within git_repository_is_empty()

Small changes enabling compiling libgit2 for Android.

Small cleanup in examples.

CMake seems not to support Android as a target and this option
lets us test this in CMakeLists.txt.

Update documentation of git_oid_streq to remove outdated error code

oid: git_oid_shorten_add() sets GITERR_INVALID when OID set is full

Update readme to point to the currently maintained Erlang bindings. 

Update readme to point to the currently maintained Erlang bindings. Namely: https://github.com/carlosmn/geef

This simplifies the git_repository_is_empty a bit so that a
detached HEAD is just taken to mean the repo is not empty, since
a newly initialized repo will not have a detached HEAD.

Reverts part of 9146f1e5.

All use of sortedcache will need this header, so put it in the
definition of the sortedcache API.

Thread safety for the refdb_fs

Even more lenient remote parsing

netops: remove duplicate include

Configuration iterators redux

Windows XP updates

In order to be loaded, a remote needs to be configured with at least a `url` or a `pushurl`.

ENOTFOUND will be returned when trying to git_remote_load() a remote with neither of these entries defined.

This loads SRWLock APIs at runtime and in their absence (i.e. on
Windows before Vista) falls back on a regular CRITICAL_SECTION
that will not permit concurrent readers.

refs: add git_reference_is_tag