Small fixes to the build notes for Mac

Use the correct git tree for libdivecomputer.
Only build libmarble, not all of marble
Don't try make install for libmarble - it's bogus
Copy the correct marble headers
Only use sudo to write into /Applications

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't set -Wno-maybe-uninitialized for clang

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>