Switch default to static linking

Build in libgit2 statically into git2go by default, removing the need
for the right version to be available as a shared object.

We do still need to link dynamically against OpenSSL and LibSSH2.

branch.go

@@ -1,7 +1,6 @@
 package git
 
 /*
-#cgo pkg-config: libgit2
 #include <git2.h>
 #include <git2/errors.h>
 */

git.go

@@ -1,7 +1,7 @@
 package git
 
 /*
-#cgo pkg-config: libgit2
+#cgo pkg-config: --static libgit2
 #include <git2.h>
 #include <git2/errors.h>
 */

settings/settings.go

@@ -1,7 +1,7 @@
 package settings
 
 /*
-#cgo pkg-config: libgit2
+#cgo pkg-config: --static libgit2
 #include <git2.h>
 
 int _go_git_opts_get_search_path(int level, git_buf *buf)