ENH: add IF(IS_ABSOLUTE path), so no regex matching is required in the cmake scripts

Alex

Source/cmIfCommand.cxx

@@ -287,6 +287,22 @@ bool cmIfCommand::IsTrue(const std::vector<std::string> &args,
         IncrementArguments(newArgs,argP1,argP2);
         reducible = 1;
         }
+      // is the given path an absolute path ?
+      if (*arg == "IS_ABSOLUTE" && argP1  != newArgs.end())
+        {
+        if(cmSystemTools::FileIsFullPath((argP1)->c_str()))
+          {
+          *arg = "1";
+          }
+        else 
+          {
+          *arg = "0";
+          }
+        newArgs.erase(argP1);
+        argP1 = arg;
+        IncrementArguments(newArgs,argP1,argP2);
+        reducible = 1;
+        }
       // does a command exist
       if (*arg == "COMMAND" && argP1  != newArgs.end())
         {

Source/cmIfCommand.h

@@ -143,6 +143,8 @@ public:
       "  IF(IS_DIRECTORY directory-name)\n"
       "True if the given name is a directory.  "
       "Behavior is well-defined only for full paths.\n"
+      "  IF(IS_ABSOLUTE path)\n"
+      "True if the given path is an absolute path.\n "
       "  IF(variable MATCHES regex)\n"
       "  IF(string MATCHES regex)\n"
       "True if the given string or variable's value matches the given "