autogen.sh: Allow running our autogen.sh out of tree

Such as:

$ mkdir -p x86_64
$ cd x86_64/
$ ../autogen.sh --prefix=/usr --enable-debug
$ make

Note that to do the above, you need to clean out the srcdir
with a 'make distclean' or 'git clean ...' first.
Makefile.am: Remove invalid -I$(top_builddir)/src directory

We don't use AM_INIT_AUTOMAKE "subdir-objects" option so ./src
does not exist as a subdirectory in builddir when doing out of
tree builds.

This fixes the warning:

cc1: warning: ./src: No such file or directory [enabled by default]
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