boost: add submodule at tag boost-1.61.0

This implements option ceph#4 for external boost, based on upstream
discussion.  In option ceph#4:

1. boost is added as a submodule
2. builds default to using the attached boost module
3. building against a system-provided boost is supported, but
   must be configured explicitly

Because all of the boost components are attached as nested
submodules in the upstream boost repository, neither the nested
submodules nor the root boost submodule have been cloned into
modules in github.com/ceph (acked by Sage).

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>

Author: mattbenjamin

.gitmodules

@@ -33,3 +33,6 @@
 [submodule "src/isa-l"]
 	path = src/isa-l
 	url = https://github.com/ceph/isa-l
+[submodule "boost"]
+	path = boost
+	url = https://github.com/boostorg/boost.git