Makefile: Change default prefix to "/usr/local"

Locally installed packages are usually installed to /usr/local.
Packagers can use `make prefix=/usr` to get back the old behavior.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

Makefile

@@ -6,7 +6,7 @@ CGIT_SCRIPT_PATH = /var/www/htdocs/cgit
 CGIT_DATA_PATH = $(CGIT_SCRIPT_PATH)
 CGIT_CONFIG = /etc/cgitrc
 CACHE_ROOT = /var/cache/cgit
-prefix = /usr
+prefix = /usr/local
 libdir = $(prefix)/lib
 filterdir = $(libdir)/cgit/filters
 docdir = $(prefix)/share/doc/cgit