configure: Include passed in cflags for ssh check
configure: Include passed in cflags for ssh check

This fixes the Fedora 23 build where the cflags and ldflags have to
match. Otherwise, if the configure check throws away cflags but uses
ldflags, the check fails with a linker error.

https://bugzilla.gnome.org/show_bug.cgi?id=747874

GNOME/libgit2-glib,GNOME/libgit2-glib