build: Makefile improvements.

[orbea]

This has various makefile usability improvements.

• The install target now depends on the all target so that the user can just type make install instead of make; make install.
• This uses a GNU make command to dynamically create the $(SRCDIR) variable so that the user can build out of tree. See the below example.
• This fixes the uninstall target by removing the bogus mkdir(1) commands and adding -f to for the rm(1) commands so that make uninstall does not fail if any of the files were manually removed prior. If the files does not exist then they won't be removed and no harm will be done.
• This adds $(BINDIR) and $(DATAROOTDIR). I also added $(DATADIR) to be complete with the GNU install directories even if its not currently all that useful for Flips.

Out of tree build example:

mkdir -p /tmp/build
cd /tmp/build
make install -f /path/to/Flips/Makefile

[Alcaro]

Looks good to me. Not sure who snuck those mkdirs past me (fair chance it was myself), good catch.