makepkg: allow specifying --pkg multiple times

Make this option additive, so that the following two operations are
equivalent:

  makepkg --pkg foo --pkg bar
  makepkg --pkg foo,bar

doc/makepkg.8.txt

@@ -154,7 +154,7 @@ Options
 
 *\--pkg <list>*::
 	Only build listed packages from a split package. Multiple packages should
-	be comma separated in the list.
+	be comma separated in the list. This option can be specified multiple times.
 
 *\--check*::
 	Run the check() function in the PKGBUILD, overriding the setting in

scripts/makepkg.sh.in

@@ -1998,7 +1998,7 @@ while true; do
 		--nosign)         SIGNPKG='n' ;;
 		-o|--nobuild)     NOBUILD=1 ;;
 		-p)               shift; BUILDFILE=$1 ;;
-		--pkg)            shift; IFS=, read -ra PKGLIST <<<"$1" ;;
+		--pkg)            shift; IFS=, read -ra p <<<"$1"; PKGLIST+=("${p[@]}"); unset p ;;
 		-r|--rmdeps)      RMDEPS=1 ;;
 		-R|--repackage)   REPKG=1 ;;
 		--skipchecksums)  SKIPCHECKSUMS=1 ;;