Makefile-web.am: Fix out of tree builds of css
Only dist and #include the libgsystem header we need

We can't distribute all of libgsystem in our tarball, since
distros start complaining.
Makefile-am: fix 'make distcheck'

Now 'make distcheck' runs tests and generates an appropriate
tarball for actual releases.
doc: Don't run inkscape for 'make all'

Add an empty all: target