build system tries to build docs from modules that are not built

[Gustavo Carneiro]

I get this for some reason that doesn't matter here:

      Build GSL module : no

I type 'make' everything builds, but at 'make install' time I get:

make[3]: *** No rule to make target `modules/gsl/gsl.xml', needed by `docs.stamp'. Stop.

This is because the gsl module was disabled, and the build system does not 'enter' the modules/gsl dir and so never builds the gsl.xml file.

This isn't very serious because only developers see this errors; other users get the pre-built gsl.xml file :)