Normalize slashes in scanned #include lines (#10281)

On Windows platforms source files may contain '\' in include directives:

  #include "a\b.h"

Normalize these while scanning to use forward slashes.  CMake will
convert from forward slashes to the direction preferred by the native
build tools when writing the path to 'depend.make' files.

Source/cmDependsC.cxx

@@ -448,6 +448,7 @@ void cmDependsC::Scan(std::istream& is, const char* directory,
       // Get the file being included.
       UnscannedEntry entry;
       entry.FileName = this->IncludeRegexLine.match(2);
+      cmSystemTools::ConvertToUnixSlashes(entry.FileName);
       if(this->IncludeRegexLine.match(3) == "\"" &&
          !cmSystemTools::FileIsFullPath(entry.FileName.c_str()))
         {