meson: Add -Werror=return-type when supported.

meson: Add -Werror=return-type when supported.

This warning detects non-void functions with a missing return statement,
return statements with a value in void functions, and functions with an
bogus return type that ends up defaulting to int.  It's already enabled
by default with -Wall.  Generally, these are fairly serious bugs in the
code, which developers would like to notice and fix immediately.  This
patch promotes it from a warning to an error, to help developers catch
such mistakes early.

I would not expect this warning to change much based on the compiler
version, so hopefully it won't become a problem for packagers/builders.

See the GCC documentation or 'man gcc' for more details:
https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/Warning-Options.html#index-Wreturn-type

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>