Record dependencies during build instead of before

Previously, the makefile created dependency files in a separate, first
pass.  In particular, include-ing the dependency files would cause
make to attempt to rebuild those files using the dependency-generation
rules in the makefile.  Unfortunately, this approach required obtuse
rules and silently delayed the start of the build process (by quite a
bit on a clean tree without any dependency files).  Worse, this
required the dependency files to themselves depend on all of the
headers the source file depended on, which meant that, if a header
file was removed, the depedency file could not be updated because of a
missing dependency (!), which would cause make to silently fail.

This patch eliminates the dependency generation rules and instead
generates dependency files as a side-effect of the regular build rule.
On the first build, we don't need to know the dependencies beforehand;
the object file doesn't exist, so it will be built anyway.  On
subsequent builds, if a header file is updated, the dependency rules
generated by the previous build will force a rebuild.  If a source
file is updated, the dependency rules may be stale, but it doesn't
matter because the updated source file will force a rebuild.

In the final case above, the stale dependency rules may refer to a
header file that no longer exists but is also no longer needed.  In
order to prevent this from breaking the build, we also pass gcc the
-MP option, which generates phony targets for every depended-on header
file, so make won't complain if it can't find them during a later
build.

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