cmCPluginAPI: Remove a few unnecessary c_str() calls

Source/cmCPluginAPI.cxx

@@ -664,7 +664,7 @@ void CCONV cmSourceFileSetName(void* arg, const char* name, const char* dir,
   // First try and see whether the listed file can be found
   // as is without extensions added on.
   std::string hname = pathname;
-  if (cmSystemTools::FileExists(hname.c_str())) {
+  if (cmSystemTools::FileExists(hname)) {
     sf->SourceName = cmSystemTools::GetFilenamePath(name);
     if (!sf->SourceName.empty()) {
       sf->SourceName += "/";
@@ -691,7 +691,7 @@ void CCONV cmSourceFileSetName(void* arg, const char* name, const char* dir,
     hname = pathname;
     hname += ".";
     hname += *ext;
-    if (cmSystemTools::FileExists(hname.c_str())) {
+    if (cmSystemTools::FileExists(hname)) {
       sf->SourceExtension = *ext;
       sf->FullPath = hname;
       return;
@@ -704,7 +704,7 @@ void CCONV cmSourceFileSetName(void* arg, const char* name, const char* dir,
     hname = pathname;
     hname += ".";
     hname += *ext;
-    if (cmSystemTools::FileExists(hname.c_str())) {
+    if (cmSystemTools::FileExists(hname)) {
       sf->SourceExtension = *ext;
       sf->FullPath = hname;
       return;