Control building documentation (BZ 495483)

Control building documentation   (BZ 495483)

Introduce Makefile variable BUILD_DOCS with these possible values:
none - does not build any documentation
all  - builds all documentation
html - builds HTML docs but skips building PDFs

BUILD_ALL_DOCS is still recognised for backward compatibility and
is mapped to BUILD_DOCS like so:
If not specified   --> BUILD_DOCS=all
BUILD_ALL_DOCS=yes --> BUILD_DOCS=all
BUILD_ALL_DOCS=no  --> BUILD_DOCS=html

README_DEVELOPERS adjusted.

Fixes https://bugs.kde.org/show_bug.cgi?id=495483