Sweep out some extra warnings

Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)

The last three are enabled for Clang compilation going forward.

Also enabled `-Wunreachable-code` for Clang and GCC

projects/SelfTest/ConditionTests.cpp

@@ -6,7 +6,9 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 #ifdef __clang__
+#   pragma clang diagnostic push
 #   pragma clang diagnostic ignored "-Wpadded"
+#   pragma clang diagnostic ignored "-Wdouble-promotion"
 #endif
 
 #include "catch.hpp"
@@ -161,6 +163,11 @@ TEST_CASE( "Ordering comparison checks that should fail", "[.][failing]" )
     CHECK( data.str_hello <= "a" );
 }
 
+#ifdef __clang__
+#   pragma clang diagnostic pop
+#endif
+
+
 // Comparisons with int literals
 TEST_CASE( "Comparisons with int literals don't warn when mixing signed/ unsigned" )
 {