find_package nanocernlib before using internal

This allows systemwide installation of nanocernlib, then cmake -Dnanocernlib_DIR=/path/to/nanocernlib/ .. 

CMakeLists.txt

@@ -31,8 +31,14 @@ add_subdirectory(pythia6m/radiator)
 add_subdirectory(pythia6m/radgen)
 add_subdirectory(pythia6m/pythia6)
 
-add_subdirectory(nanocernlib)
-
+find_package(nanocernlib)
+if(NOT ${nanocernlib_LIBRARIES})
+  message(STATUS "Using internal nanocernlib")
+  add_subdirectory(nanocernlib)
+else()
+  message(STATUS "Using nanocernlib at ${NANOCERNLIB_INCLUDE_DIR}")
+endif()
+ 
 add_subdirectory(program)
 
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