coments to linker warnings on macOS/Clang added

CMakeLists.txt

@@ -68,13 +68,17 @@ elseif (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
     add_compile_options (-Warray-bounds)
     # Increase template depth for boost
     add_compile_options (-ftemplate-depth=1024)
+    # get rid of linking warning with boost. Looks like that boost
+    # is using this visibility setting.
     add_compile_options (-fvisibility=hidden)
     add_compile_options (-fvisibility-inlines-hidden)
     # Allow overloaded virtual functions (instances to be fixed in OPAL)
     #add_compile_options (-Wno-overloaded-virtual)
     if (ENABLE_OpenMP)
-	    add_compile_options (-fopenmp)
+	add_compile_options (-fopenmp)
     endif ()
+    # Not 100% clear why we need this. But without we get warning while
+    # linking with HDF5
     add_link_options (-Wl,-no_compact_unwind)
 
 elseif (${CMAKE_CXX_COMPILER_ID} MATCHES "GNU")