Installing boost-spirit "properly"

Boost-spirit is not a self-contained library by any means,
unfortunately. Rather than inventing ad-hoc measures of installing the
dependencies and dependencies of dependencies and..., let's stop playing
whack-a-mole and instead use the big installation hammer.

We're not calling `b2 install` because that one attempts to build these
libraries. We do not want that. On the other hand, `b2 headers` is
"lazy" as it prefers to create symlinks where it makes sense. These
symlinks are relative, pointing to the source directory of each
individual Boost module -- hence that `cp`.

Yay for custom build systems, once again :(.

Change-Id: Iad867386ac5907630db8bdde0668d8fd9edee580
diff --git a/.gitmodules b/.gitmodules
index 6fa00a0..00eeb89 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -4,12 +4,12 @@
 [submodule "submodules/trompeloeil"]
 	path = submodules/trompeloeil
 	url = ../../github/rollbear/trompeloeil
-[submodule "submodules/spirit"]
-	path = submodules/spirit
-	url = ../../github/boostorg/spirit
 [submodule "submodules/docopt.cpp"]
 	path = submodules/docopt.cpp
 	url = ../../github/docopt/docopt.cpp
 [submodule "submodules/spdlog"]
 	path = submodules/spdlog
 	url = ../../github/gabime/spdlog
+[submodule "submodules/boost"]
+	path = submodules/boost
+	url = ../../github/boostorg/boost