Look for alternative implementations of 7zip, like autoconf did

Fixes a regression from the cmake transition where we'd just assume 7zip
as the name of the executable when previously we looked through 7zip,
7za and 7a. Resume the former behavior.

Fixes: rpm-software-management#2608

CMakeLists.txt

@@ -88,7 +88,7 @@ function(makemacros)
 	set(rundir /run)
 	set(root_prefix /usr)
 
-	findutil(__7ZIP 7zip)
+	findutil(__7ZIP "7zip;7za;7z")
 	findutil(__BZIP2 bzip2)
 	findutil(__CAT cat)
 	findutil(__CHMOD chmod)