Add support for libgit2 feature detection

This lets us ask the library whether it supports threading, https and
ssh.

docs/features.rst

@@ -0,0 +1,8 @@
+**********************************************************************
+Feature detection
+**********************************************************************
+
+.. py:data:: pygit2.features
+
+   This variable contains a combination of `GIT_FEATURE_*` flags,
+   indicating which features a particular build of libgit2 supports.

docs/index.rst

@@ -47,6 +47,7 @@ Usage guide:
    remotes
    blame
    settings
+   features
 
 
 Indices and tables

pygit2/__init__.py

@@ -221,3 +221,8 @@ def clone_into(repo, remote, branch=None):
     check_error(err)
 
 settings = Settings()
+
+features = C.git_libgit2_features()
+GIT_FEATURE_THREADS = C.GIT_FEATURE_THREADS
+GIT_FEATURE_HTTPS = C.GIT_FEATURE_HTTPS
+GIT_FEATURE_SSH = C.GIT_FEATURE_SSH

pygit2/decl.h

@@ -74,6 +74,12 @@ typedef struct git_signature {
 	git_time when;
 } git_signature;
 
+#define GIT_FEATURE_THREADS ...
+#define GIT_FEATURE_HTTPS ...
+#define GIT_FEATURE_SSH ...
+
+int git_libgit2_features(void);
+
 const git_error * giterr_last(void);
 
 void git_strarray_free(git_strarray *array);