Suppress clang constant-logical-operand warning.

This is only triggered on some minilzo.cpp code with a particular
set of detected features. In those cases the code is redundant
and will be optimized away, in other cases it isn't so we don't
want to remove the code.

mythtv/configure

@@ -4750,6 +4750,10 @@ elif enabled clangxx; then
     # The self assign warning triggers on headers that use x=x;
     # to suppress warnings about unused parameters.
     check_cxxflags -Wno-self-assign
+    # The constant-logical-operand only gives use false positives
+    # for constant logical operands meant to be optimized away.
+    check_cxxflags -Wno-constant-logical-operand
+    # clang complains about every unused -I unless you pass it this.
     check_cxxflags -Qunused-arguments
 fi