BUG: Remove implicit include dir suppression

We used to suppress generation of -I/usr/include (and on OSX also
-I/usr/local/include).  This behavior seems to cause more trouble than
it's worth, so I'm removing it until someone encounters the original
problem it fixed.  See issue #8598.

Source/cmLocalGenerator.h

@@ -189,8 +189,7 @@ public:
     { return true; }
 
   /** Get the include flags for the current makefile and language.  */
-  void GetIncludeDirectories(std::vector<std::string>& dirs,
-                             bool filter_system_dirs = true);
+  void GetIncludeDirectories(std::vector<std::string>& dirs);
 
   /** Compute the language used to compile the given source file.  */
   const char* GetSourceFileLanguage(const cmSourceFile& source);