cmake: drop -rdynamic aka CMP0065 NEW

Prior to version 3.3 cmake would always use -rdynamic. That in itself
causes all the internal symbols to be exported, increasing the binaries
by 5-10% and making it impossible for the compiler to reason, optimise
and discard unused code.

The -rdynamic is useful in two cases:
 - having a third party module (say /usr/lib/foo/foobar.so) which is
   underlinked and depends on symbols from the main binary - apps like
   irssi, bash and zsh use that

 - uses the glibc backtrace, which relies on dlopen/dlsym to fetch the
   symbol data. Unwind is much better solution, since it replies on the
   DWARF data

Our binaries do not use either of these - so drop the -rdynamic. The
autotools build doesn't use it either.

   text    data     bss     dec     hex filename
 229000    2120    4424  235544   39818 bsdcat -- before
 208324    2120    4424  214868   34754 bsdcat -- after
1093939   12128   24176 1130243  113f03 bsdcpio -- before
1059181   12128   24176 1095485  10b73d bsdcpio -- after
1130091   14264    6608 1150963  118ff3 bsdtar -- before
1093690   14264    6608 1114562  1101c2 bsdtar -- after

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

CMakeLists.txt

@@ -1,5 +1,8 @@
 #
 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12 FATAL_ERROR)
+if(POLICY CMP0065)
+  cmake_policy(SET CMP0065 NEW) #3.4 don't use `-rdynamic` with executables
+endif()
 if(POLICY CMP0074)
   cmake_policy(SET CMP0074 NEW) #3.12.0 `find_package()`` uses ``<PackageName>_ROOT`` variables.
 endif()