Ensure vertical_EAS is only built if Pythia is available.

The `vertical_EAS` example requires Pythia however the build process
still tried to compile the example code even when Pythia was
unavailable. This adds an explicit dependence on Pythia for the
vertical air shower example.

Documentation/Examples/CMakeLists.txt

@@ -85,33 +85,34 @@ if (Pythia8_FOUND)
     CORSIKAprocesssequence
     )
   install (TARGETS cascade_proton_example DESTINATION share/examples)
-endif()
 
-add_executable(vertical_EAS vertical_EAS.cc)
-target_link_libraries (vertical_EAS
-  SuperStupidStack
-  CORSIKAunits
-  CORSIKAlogging
-  CORSIKArandom
-  ProcessSibyll
-  ProcessPythia
-  ProcessUrQMD
-  ProcessSwitch
-  CORSIKAcascade
-  ProcessEnergyLoss
-  ProcessObservationPlane
-  ProcessTrackingLine
-  ProcessParticleCut
-  ProcessStackInspector
-  ProcessInteractionCounter
-  CORSIKAprocesses
-  CORSIKAcascade
-  CORSIKAparticles
-  CORSIKAgeometry
-  CORSIKAenvironment
-  CORSIKAprocesssequence
-  )
-install (TARGETS vertical_EAS DESTINATION share/examples)
+  add_executable(vertical_EAS vertical_EAS.cc)
+  target_link_libraries (vertical_EAS
+    SuperStupidStack
+    CORSIKAunits
+    CORSIKAlogging
+    CORSIKArandom
+    ProcessSibyll
+    ProcessPythia
+    ProcessUrQMD
+    ProcessSwitch
+    CORSIKAcascade
+    ProcessEnergyLoss
+    ProcessObservationPlane
+    ProcessTrackingLine
+    ProcessParticleCut
+    ProcessStackInspector
+    ProcessInteractionCounter
+    CORSIKAprocesses
+    CORSIKAcascade
+    CORSIKAparticles
+    CORSIKAgeometry
+    CORSIKAenvironment
+    CORSIKAprocesssequence
+    )
+  install (TARGETS vertical_EAS DESTINATION share/examples)
+  
+endif()
 
 CORSIKA_ADD_TEST(stopping_power)
 target_link_libraries (stopping_power
@@ -123,7 +124,6 @@ target_link_libraries (stopping_power
   CORSIKAenvironment
   )
 
-install (TARGETS vertical_EAS DESTINATION share/examples)
 CORSIKA_ADD_TEST (staticsequence_example)
 target_link_libraries (staticsequence_example
   CORSIKAprocesssequence