use size_t for GeneratorExpressionContent::ContentLength to fix some warnings

CMake/Source/cmGeneratorExpressionParser.cxx: In member function ‘void cmGeneratorExpressionParser::ParseGeneratorExpression(std::vector<cmGeneratorExpressionEvaluator*>&)’:
CMake/Source/cmGeneratorExpressionParser.cxx:116:55: warning: conversion to ‘unsigned int’ from ‘long int’ may alter its value [-Wconversion]
CMake/Source/cmGeneratorExpressionParser.cxx:240:39: warning: conversion to ‘int’ from ‘long int’ may alter its value [-Wconversion]

Source/cmGeneratorExpressionEvaluator.cxx

@@ -1446,7 +1446,7 @@ cmGeneratorExpressionNode* GetNode(const std::string &identifier)
 //----------------------------------------------------------------------------
 GeneratorExpressionContent::GeneratorExpressionContent(
                                                     const char *startContent,
-                                                    unsigned int length)
+                                                    size_t length)
   : StartContent(startContent), ContentLength(length)
 {
 

Source/cmGeneratorExpressionEvaluator.h

@@ -63,7 +63,7 @@ private:
 
 struct TextContent : public cmGeneratorExpressionEvaluator
 {
-  TextContent(const char *start, unsigned int length)
+  TextContent(const char *start, size_t length)
     : Content(start), Length(length)
   {
 
@@ -80,25 +80,25 @@ struct TextContent : public cmGeneratorExpressionEvaluator
     return cmGeneratorExpressionEvaluator::Text;
   }
 
-  void Extend(unsigned int length)
+  void Extend(size_t length)
   {
     this->Length += length;
   }
 
-  unsigned int GetLength()
+  size_t GetLength()
   {
     return this->Length;
   }
 
 private:
   const char *Content;
-  unsigned int Length;
+  size_t Length;
 };
 
 //----------------------------------------------------------------------------
 struct GeneratorExpressionContent : public cmGeneratorExpressionEvaluator
 {
-  GeneratorExpressionContent(const char *startContent, unsigned int length);
+  GeneratorExpressionContent(const char *startContent, size_t length);
   void SetIdentifier(std::vector<cmGeneratorExpressionEvaluator*> identifier)
   {
     this->IdentifierChildren = identifier;
@@ -141,7 +141,7 @@ private:
   std::vector<cmGeneratorExpressionEvaluator*> IdentifierChildren;
   std::vector<std::vector<cmGeneratorExpressionEvaluator*> > ParamChildren;
   const char *StartContent;
-  unsigned int ContentLength;
+  size_t ContentLength;
 };
 
 #endif

Source/cmGeneratorExpressionLexer.h

@@ -19,7 +19,7 @@
 //----------------------------------------------------------------------------
 struct cmGeneratorExpressionToken
 {
-  cmGeneratorExpressionToken(unsigned type, const char *c, unsigned l)
+  cmGeneratorExpressionToken(unsigned type, const char *c, size_t l)
     : TokenType(type), Content(c), Length(l)
   {
   }
@@ -32,7 +32,7 @@ struct cmGeneratorExpressionToken
   };
   unsigned TokenType;
   const char *Content;
-  unsigned Length;
+  size_t Length;
 };
 
 /** \class cmGeneratorExpressionLexer

Source/cmGeneratorExpressionParser.cxx

@@ -235,7 +235,7 @@ void cmGeneratorExpressionParser::ParseGeneratorExpression(
     return;
   }
 
-  int contentLength = ((this->it - 1)->Content
+  size_t contentLength = ((this->it - 1)->Content
                     - startToken->Content)
                     + (this->it - 1)->Length;
   GeneratorExpressionContent *content = new GeneratorExpressionContent(