CTestCustom.cmake: Ignore clang's summary warning

Assuming prior text has matched some other warning expression...

CTestCustom.cmake.in

@@ -44,6 +44,10 @@ SET(CTEST_CUSTOM_WARNING_EXCEPTION
   "cc-3968 CC: WARNING File.*" # "implicit" truncation by static_cast
   "ld: warning: directory not found for option .-(F|L)"
   "warning.*This version of Mac OS X is unsupported"
+
+  # Ignore clang's summary warning, assuming prior text has matched some
+  # other warning expression:
+  "[0-9,]+ warnings? generated."
   )
 
 IF(NOT "@CMAKE_GENERATOR@" MATCHES "Xcode")