When MaxCtrl is being built, the source is copied into the build directory
to prevent polluting the source tree with node_modules. This means that
any relative references MaxCtrl builds make outside of the maxctrl
directory must be copied to the build directory.
The internal header directory conflicted with in-source builds causing a
build failure. This is fixed by renaming the internal header directory to
something other than maxscale.
The renaming pointed out a few problems in a couple of source files that
appeared to include internal headers when the headers were in fact public
headers.
Fixed maxctrl in-source builds by making the copying of the sources
optional.
