configure: make assignment of variable to '=' explicit

op== is confusing, as it may easily be mistook for an equality check,
while in reality it is doing an assigment to '='.
This patch has the additional benefit of cleaning up 2 warnings from shellcheck.net.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

configure

@@ -622,12 +622,12 @@ enable_deep_weak(){
 }
 
 enabled(){
-    test "${1#!}" = "$1" && op== || op=!=
+    test "${1#!}" = "$1" && op='=' || op=!=
     eval test "x\$${1#!}" $op "xyes"
 }
 
 disabled(){
-    test "${1#!}" = "$1" && op== || op=!=
+    test "${1#!}" = "$1" && op='=' || op=!=
     eval test "x\$${1#!}" $op "xno"
 }