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>

meson.build

@@ -788,7 +788,8 @@ endif
 # Check for generic C arguments
 c_args = []
 foreach a : ['-Wall', '-Werror=implicit-function-declaration',
-             '-Werror=missing-prototypes', '-fno-math-errno',
+             '-Werror=missing-prototypes', '-Werror=return-type',
+             '-fno-math-errno',
              '-fno-trapping-math', '-Qunused-arguments']
   if cc.has_argument(a)
     c_args += a
@@ -808,7 +809,8 @@ endif
 
 # Check for generic C++ arguments
 cpp_args = []
-foreach a : ['-Wall', '-fno-math-errno', '-fno-trapping-math',
+foreach a : ['-Wall', '-Werror=return-type',
+             '-fno-math-errno', '-fno-trapping-math',
              '-Qunused-arguments']
   if cpp.has_argument(a)
     cpp_args += a