Revert "Add REQUIRED flag to dependencies that cannot be built from source"

This reverts commit 0589bd45.

CMakeLists.txt

@@ -24,7 +24,7 @@ option(WITH_SHARED_MEMORY "Include the shared memory transport protocol" ON)
 include(ExternalDependency)
 
 if (WITH_SHARED_MEMORY)
-  find_package(Boost REQUIRED)
+  find_package(Boost)
   if (Boost_FOUND)
     add_dependency_entry(
       NAME Boost
@@ -142,7 +142,7 @@ if (BUILD_NEST)
     MESSAGE "will be build from source"
   )
 
-  find_package(Cython REQUIRED)
+  find_package(Cython)
   if (CYTHON_FOUND)
     add_dependency_entry(
       NAME Cython
@@ -157,7 +157,7 @@ if (BUILD_NEST)
     )
   endif ()
 
-  find_package(OpenMP REQUIRED)
+  find_package(OpenMP)
   if (OPENMP_FOUND)
     add_dependency_entry(
       NAME OpenMP