makepkg: fix initialization when extracting arrays

Assuming that everything is a string leads to code which is effectively:

  a=
  a+=('bar')

This creates an array with 2 elements instead of one. Using proper array
initialization fixes this.

https://lists.archlinux.org/pipermail/pacman-dev/2018-June/022591.html
Signed-off-by: Allan McRae <allan@archlinux.org>

scripts/libmakepkg/util/pkgbuild.sh.in

@@ -106,7 +106,11 @@ get_pkgbuild_attribute() {
 
 	local pkgname=$1 attrname=$2 isarray=$3 outputvar=$4
 
+	if (( isarray )); then
+		eval "$outputvar=()"
+	else
 		printf -v "$outputvar" %s ''
+	fi
 
 	if [[ $pkgname ]]; then
 		extract_global_variable "$attrname" "$isarray" "$outputvar"