Makefile: build the "all" target by default without examples or docs

Previously, only the "libiio.so.0.1" target would be built by default
causing the standard `make` and then `make install` commands to perform
compiling/linking during the install phase since iiod and tests weren't
built yet.

The examples and html docs have been removed from the default target
since neither appears to be installed by default.

Signed-off-by: Tim Harder <timothy.harder@analog.com>