gtk-doc.make: substitude exit code correctly

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gtk-doc.make: gather generic bits

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gtk-doc.make: move similar rules together

This patch moves several definitions around so that it's possible to
group rules for same DOC_MODULE together.

Besides, this patch also eliminates the use of HTML_BUILD_STAMP and
PDF_BUILD_STAMP. They're barely helpful, and by replacing them with
hard coded names, it will be easier to convert that Makefile to
support subdir-objects.

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gtk-doc.make: add prefix to module variables

In order to support subdir-objects style Makefile, gtk-doc.make
should be able to handle multiple doc module definitions, which
follows there must be a way to separate variables of different
modules. In this patch we turn most lines into Makefile funtions and
therefore gain the ability to evaluate them with a prefix parameter.

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
gtk-doc.make: subdir-object support

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>