systemd: fix build with gcc <= 4.7

Pass -Werror=shadow in args of cc.compiles in meson.build otherwise test
will always succeed, causing -Werror=shadow to be passed, even on older gcc versions.

GCC 4.8 changed the behaviour of -Werror=shadow to no longer complain about
local variable declariations shadowing functions, which systemd has.  From
the changelog:

  The option -Wshadow no longer warns if a declaration shadows a function
  declaration, unless the former declares a function or pointer to function,
  because this is a common and valid case in real-world code.

https://www.gnu.org/software/gcc/gcc-4.8/changes.html

Fixes:
 - http://autobuild.buildroot.org/results/ffd71c473d3b29618c18cd2e04705370266696f2



[Peter: extend commit message, add gcc 4.8 link]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>