makepkg: Empty/create only $pkgdir's relevant to current PKGBUILD

Currently makepkg clears the whole $pkgbasedir which is needless. Moreover,
in the obscure case of multiple makepkg runs (with different $pkgname) that
share a $pkgdirbase, only $pkgdir's from the last run will remain. Since
I consider the contents of $pkgdir an important artifact, this commit restricts
the deletion to individual $pkgdir's.

When CLEANUP is set, the behavior is unchanged.

Discussed in:
https://lists.archlinux.org/pipermail/pacman-dev/2015-February/019939.html

Signed-off-by: Allan McRae <allan@archlinux.org>

scripts/makepkg.sh.in

@@ -3413,10 +3413,13 @@ if (( NOBUILD )); then
 	msg "$(gettext "Sources are ready.")"
 	exit 0 #E_OK
 else
-	# clean existing pkg directory
+	# clean existing pkg directories
 	if [[ -d $pkgdirbase ]]; then
 		msg "$(gettext "Removing existing %s directory...")" "\$pkgdir/"
-		rm -rf "$pkgdirbase"
+		for pkg in "${pkgname[@]}"; do
+			rm -rf "$pkgdirbase/$pkg"
+		done
+		unset pkg
 	fi
 	mkdir -p "$pkgdirbase"
 	chmod a-srwx "$pkgdirbase"