build: add PCHs for ndnsec and unit tests, fine-tune the existing ones

A full debug+tests build now takes 10% less time with gcc-9 and 21% less
with clang-10. Release builds (without tests) are only marginally faster
with gcc and 7% faster with clang.

Change-Id: I494778fe44cecc6209819a49f48a03f4d16c36af
diff --git a/wscript b/wscript
index 3765aa4..1c536d1 100644
--- a/wscript
+++ b/wscript
@@ -181,7 +181,7 @@
 
     if bld.env.HAVE_OSX_FRAMEWORKS:
         # Need to disable precompiled headers for Objective-C++ code
-        bld(features=['cxx'],
+        bld(features='cxx',
             target='ndn-cxx-mm-objects',
             source=bld.path.ant_glob('ndn-cxx/**/*-osx.mm'),
             use='BOOST PTHREAD OSX_COREFOUNDATION OSX_SECURITY OSX_SYSTEMCONFIGURATION OSX_FOUNDATION OSX_COREWLAN',