Add boolean generator expressions

Add generator expressions that combine and use boolean test results:

 $<0:...>         = empty string (ignores "...")
 $<1:...>         = content of "..."
 $<AND:?[,?]...>  = '1' if all '?' are '1', else '0'
 $<OR:?[,?]...>   = '0' if all '?' are '0', else '1'
 $<NOT:?>         = '0' if '?' is '1', else '1'

These will be useful to evaluate (future) boolean query expressions and
condition content on the results.  Include tests and documentation.

Source/cmGeneratorExpression.cxx

@@ -102,6 +102,26 @@ bool cmGeneratorExpression::Evaluate()
   return true;
 }
 
+//----------------------------------------------------------------------------
+static bool cmGeneratorExpressionBool(const char* c, std::string& result,
+                                      const char* name,
+                                      const char* a, const char* b)
+{
+  result = a;
+  while((c[0] == '0' || c[0] == '1') && (c[1] == ',' || c[1] == '>'))
+    {
+    if(c[0] == b[0]) { result = b; }
+    c += 2;
+    }
+  if(c[0])
+    {
+    result = name;
+    result += " requires one or more comma-separated '0' or '1' values.";
+    return false;
+    }
+  return true;
+}
+
 //----------------------------------------------------------------------------
 bool cmGeneratorExpression::Evaluate(const char* expr, std::string& result)
 {
@@ -116,6 +136,32 @@ bool cmGeneratorExpression::Evaluate(const char* expr, std::string& result)
     {
     result = this->Config? this->Config : "";
     }
+  else if(strncmp(expr, "$<0:",4) == 0)
+    {
+    result = "";
+    }
+  else if(strncmp(expr, "$<1:",4) == 0)
+    {
+    result = std::string(expr+4, strlen(expr)-5);
+    }
+  else if(strncmp(expr, "$<NOT:",6) == 0)
+    {
+    const char* c = expr+6;
+    if((c[0] != '0' && c[0] != '1') || c[1] != '>' || c[2])
+      {
+      result = "NOT requires exactly one '0' or '1' value.";
+      return false;
+      }
+    result = c[0] == '1'? "0" : "1";
+    }
+  else if(strncmp(expr, "$<AND:",6) == 0)
+    {
+    return cmGeneratorExpressionBool(expr+6, result, "AND", "1", "0");
+    }
+  else if(strncmp(expr, "$<OR:",5) == 0)
+    {
+    return cmGeneratorExpressionBool(expr+5, result, "OR", "0", "1");
+    }
   else
     {
     result = "Expression syntax not recognized.";