fix: Make test executables optional

When including zlib as a submodule, it is useful to be able to disable the tests.

CMakeLists.txt

@@ -193,10 +193,8 @@ endif()
 #============================================================================
 # Example binaries
 #============================================================================
-
 add_executable(example test/example.c)
 target_link_libraries(example zlib)
-add_test(example example)
 
 add_executable(minigzip test/minigzip.c)
 target_link_libraries(minigzip zlib)
@@ -205,9 +203,16 @@ if(HAVE_OFF64_T)
     add_executable(example64 test/example.c)
     target_link_libraries(example64 zlib)
     set_target_properties(example64 PROPERTIES COMPILE_FLAGS "-D_FILE_OFFSET_BITS=64")
-    add_test(example64 example64)
 
     add_executable(minigzip64 test/minigzip.c)
     target_link_libraries(minigzip64 zlib)
     set_target_properties(minigzip64 PROPERTIES COMPILE_FLAGS "-D_FILE_OFFSET_BITS=64")
 endif()
+
+if(BUILD_TESTING)
+    add_test(example example)
+
+    if(HAVE_OFF64_T)
+        add_test(example64 example64)
+    endif()
+endif()