makepkg: print "SKIP" for signature checksums

There are valid reasons for a source files PGP signature to be changed
(expired key, expired signature, additional person signing...). Thus
providing a checksum for signature files can potentially cause a
PKGBUILD to require unnecessary updating.

Avoid this issue by using "SKIP" for the signature checksum.
Fixes FS#31590.

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

scripts/makepkg.sh.in

@@ -1049,10 +1049,14 @@ generate_checksums() {
 					sum="SKIP"
 					;;
 				*)
-					local file
-					file="$(get_filepath "$netfile")" || missing_source_file "$netfile"
-					sum="$(openssl dgst -${integ} "$file")"
-					sum=${sum##* }
+					if [[ ! $netfile = *.@(sig?(n)|asc) ]]; then
+						local file
+						file="$(get_filepath "$netfile")" || missing_source_file "$netfile"
+						sum="$(openssl dgst -${integ} "$file")"
+						sum=${sum##* }
+					else
+						sum="SKIP"
+					fi
 					;;
 			esac