Avoid compiler warning in rootcling and rootcling_stage1

Warning:
root/core/rootcling_stage1/src/rootcling_stage1.cxx(38):
	warning root-project#69: integer conversion resulted in truncation

   auto dummyVal = (int)(long)&usedToIdentifyRootClingByDlSym;
                    ^^^

The method above uses a cast to long, followed by a cast to int,
which results in a truncation. That is harmless, since the value
is never used, but generates a compiler warning with ICC 17. This
commit avoids the warning by storing the address of the same symbol
in a static variable instead.

core/rootcling_stage1/src/rootcling_stage1.cxx

@@ -17,6 +17,8 @@ extern "C" {
    R__DLLEXPORT void usedToIdentifyRootClingByDlSym() {}
 }
 
+// force compiler to emit symbol for function above
+static void (*dlsymaddr)() = &usedToIdentifyRootClingByDlSym;
 
 ROOT::Internal::RootCling::TROOTSYSSetter gROOTSYSSetter;
 
@@ -32,12 +34,7 @@ static const char *GetEtcDir() {
 
 int main(int argc, char **argv)
 {
-   // Force the emission of the symbol - the compiler cannot know that argv
-   // is always set.
-   if (!argv) {
-      auto dummyVal =  (int)(long)&usedToIdentifyRootClingByDlSym;
-      return dummyVal;
-   }
+   (void) dlsymaddr; // avoid unused variable warning
 
    ROOT::Internal::RootCling::DriverConfig config{};
 

main/src/rootcling.cxx

@@ -19,14 +19,12 @@ extern "C" {
    R__DLLEXPORT void usedToIdentifyRootClingByDlSym() {}
 }
 
+// force compiler to emit symbol for function above
+static void (*dlsymaddr)() = &usedToIdentifyRootClingByDlSym;
+
 int main(int argc, char **argv)
 {
-   // Force the emission of the symbol - the compiler cannot know that argv
-   // is always set.
-   if (!argv) {
-      auto dummyVal =  (int)(long)&usedToIdentifyRootClingByDlSym;
-      return dummyVal;
-   }
+   (void) dlsymaddr; // avoid unused variable warning
 
    ROOT::Internal::RootCling::DriverConfig config{};