does our cmake support really not use separate build dirs?

[Greg Troxel <gdt%lexort.com@localhost>]

I'm trying to update something, and it's moved from autoconf to cmake :-(

Adding USE_CMAKE=yes, I get a complaint that in-source builds are not
supported.  Which is a bit much of upstream, but on the other hand
nobody should be doing that either.

Reading the documentation at mk/configure/cmake.mk, I don't see how it
says that it always creates a "build" dir and uses it, or how to
enable/disable that kind of behavior.  This feels like a bug (but I also
feel unclear).

Grepping around, I find in one package:

  post-extract:
          ${RUN} mkdir -p ${WRKSRC}/build
  CONFIGURE_DIRS= build
  CMAKE_ARG_PATH= ${WRKSRC}

which is kind of icky but I suppose will work.

It seems that cmake.mk should either have a build dir variable, or a
CMAKE_USE_BUILD_DIR defaulting to true that causes this behavior.  And
that many others before me would have been running into this.