Install a shared library

This required a fair bit of surgery because previously ostree.h
included otutil.h, but that's supposed to be a private library.

src/libostree/ostree-1.pc.in

@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: OSTree
+Description: Git for operating system binaries
+Version: @VERSION@
+Requires: gio-unix-2.0
+Libs: -L${libdir} -lostree-1
+Cflags: -I${includedir}/ostree-1