Avoid rebuilding of doc, (mem)test and install targets

Avoid rebuilding of doc, (mem)test and install targets

This patch improves the Makefiles to avoid unnecessary rebuilding of some
targets, like doc/test/memtest/install, which only need to be "rebuilt"
when the sources has changed.

This is implemented using empty target files to record events, as
described in the GNU make manual[1].

[1] http://www.gnu.org/software/make/manual/make.html#Empty-Targets