gc.sh, utils/generate-db-build.sh: follow symlinks in build/*.
utils/generate-makefile.sh: prefer building over installing from repos.

Thus we can build packages with same name as officials.
utils/generate-makefile.sh: use `realpath --relative-to` instrad of `basename` on realpath result.
utils/prepare-mode.sh: always export MAKEPKG_CONFIG.

It does not hurt.
sync.sh: fix usage text.
sync.sh: add -b/--db flags for creating a pacman database in PKGDEST after building.