Add CMake target for every remaining library (#263)

This should, in principle, enable a full Au installation via CMake!

I took a more expansive view of which libraries are "for public
consumption" relative to bazel.  The old division was based on a strong
expectation that users would always want to include one or at most a
very few files, before we fully appreciated the benefits of file-by-file
inclusion for build speed.  I'll bring bazel in line in a follow up PR.

In making these CMake targets, I discovered two small irregularities in
their bazel counterparts --- namely, a missing dependency, and a bizarre
`copts` line that I think came from a stray default vim snippet.  I
fixed them in this same PR.

Helps #215.  In order to resolve it fully, we'll need to tidy up our
documentation posture w.r.t. CMake.

au/BUILD.bazel

@@ -70,6 +70,7 @@ cc_library(
         ":io",
         ":stdx",
         ":unit_of_measure",
+        "@com_google_googletest//:gtest",
     ],
 )
 
@@ -123,7 +124,6 @@ cc_test(
     name = "apply_magnitude_test",
     size = "small",
     srcs = ["apply_magnitude_test.cc"],
-    copts = ["-Iexternal/gtest/include"],
     deps = [
         ":apply_magnitude",
         ":testing",