use proper STREQUAL instead of EQUAL to compare strings

Author: Alexander Stefanov

CMakeLists.txt

@@ -492,7 +492,7 @@ execute_process(
 if(CACHELINE_SIZE STREQUAL ""
    OR CACHELINE_SIZE EQUAL 0
    OR CACHELINE_SIZE EQUAL -1
-   OR CACHELINE_SIZE EQUAL "undefined")
+   OR CACHELINE_SIZE STREQUAL "undefined")
   set(ZMQ_CACHELINE_SIZE 64)
 else()
   set(ZMQ_CACHELINE_SIZE ${CACHELINE_SIZE})