MCOL-4738 AVG() returns a wrong result

On AMD64 machines, the fpu is 80 bits. The unused bits must be masked for memcmp to work properly. For other archetectures, we don't want to mask those bits.

cmake/configureEngine.cmake

@@ -6,6 +6,7 @@
 INCLUDE (CheckIncludeFiles)
 INCLUDE (CheckIncludeFileCXX)
 INCLUDE (CheckCSourceCompiles)
+INCLUDE (CheckCXXSourceRuns)
 INCLUDE (CheckCXXSourceCompiles)
 INCLUDE (CheckStructHasMember)
 INCLUDE (CheckLibraryExists)
@@ -719,3 +720,20 @@ EXECUTE_PROCESS(
     COMMAND rm -f conftest.data conftest.file conftest.sym
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
     )
+    
+CHECK_CXX_SOURCE_RUNS("
+#include <limits>
+int main()
+{
+    // If long double is 16 bytes and digits and exponent are 64 and 16384 respectively, then we need to mask out the 
+    // unused bits, as they contain garbage. There are times we test for equality by memcmp of a buffer containing,
+    // in part, the long double set here. Garbage bytes will adversly affect that compare.
+    // Note: There may be compilers that store 80 bit floats in 12 bytes. We do not account for that here. I don't believe
+    // there are any modern Linux compilers that do that as a default. Windows uses 64 bits, so no masking is needed.
+    if (std::numeric_limits<long double>::digits == 64 
+     && std::numeric_limits<long double>::max_exponent == 16384 
+     && sizeof(long double) == 16)
+        return 0;
+    return 1; 
+}"
+MASK_LONGDOUBLE)