Use "/usr/local" as default prefix for "make install" (#448)
Use "/usr/local" as default prefix for "make install" (#448)

* Use "/usr/local" as default prefix for "make install"

Convential filesystem standards imply that "/usr" prefix should be
used only by system package managers. Custom installations should go
to /usr/local by default.

However, we *should* keep "/usr" prefix for packaging, therefore
override the default for "deb" and "rpm" targets.

* Print out installation paths

It's better for our users to know where do we install files to and
where do we remove them from during uninstallation. Let's print the
PREFIX value for "make install" and "make uninstall" targets.

* Warning about multiple Themis installations

Changing default installation path from /usr to /usr/local is more or
less safe on Linux as the compilers and linker-loaders *usually* check
/usr/local before /usr. Therefore after "sudo make install" the software
will (probably) be using the new version of Themis. However, the old
installation may still remain there in "/usr" and this may cause issues
with non-standard configurations.

Check if there are multiple installations of Themis and print a warning
for the users, suggesting to keep only one installation in their system.
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