Fix or cast integer conversions in cmake

These were revealed by GCC's -Wconversion option.  Fix types where it is
easy to do so.  Cast in cases we know the integer will not be truncated.

Source/cmStringCommand.cxx

@@ -739,7 +739,7 @@ bool cmStringCommand
     alphabet = cmStringCommandDefaultAlphabet;
     }
 
-  double sizeofAlphabet = alphabet.size();
+  double sizeofAlphabet = static_cast<double>(alphabet.size());
   if ( sizeofAlphabet < 1 )
     {
     this->SetError("sub-command RANDOM invoked with bad alphabet.");