Rationalize PHONY target and other targets

Many target's names change with this commit in order to make to know
when to generate an output file and when it's up-to-date. Better
dependency is also achieved by giving good target names.
Update INSTALL.txt file

An update for matching the new way of generating library and binaries.
Update README.txt file

An update for matching the new way of generating library and binaries.
Change some OSX parameters

Targets shouldn't contain OS specifics statements - it's not even nice
there.
Also, fix the LHARDEN flags for OSX ld vs GNU ld.
Change the way snapshots are done

There's no need to commit everything to use snapshots so just remove
this part from the snapshot target and build the tar archive anyway.
Change flags for dynamic shared object on OSX

Dynamic shared objects aren't created in the same way in OSX as in
Linux. Therefore, change a few variables to match the flags a dynamic
library should use on OSX.