build: add "-Wvla" to warn about uses of variable-length arrays

We don't use them, so add a compiler warning about their use.
I think they are annoying, because sizeof(x) and typeof(x) might
evaluate at runtime. Especially the typeof() extension is useful
for macros, but behaves badly with variable-length arrays due to
running code at runtime.

But the worst is, G_STATIC_ASSERT() is implemented by declaring
an array of negative length. Usually, the checked condition should
be a compile time constant, but with VLAs the compiler would not evaluate
the static-assert at compile time and instead accept it silently. It's easy
to mess up static asserts to wrongly have a non-constant condition,
especially when the static-assert is used inside a macro.

Just say no.

m4/compiler_options.m4

@@ -79,6 +79,7 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wshift-negative-value \
 		      -Wstrict-prototypes \
 		      -Wundef \
+		      -Wvla \
 		      -Wno-duplicate-decl-specifier \
 		      -Wno-format-truncation \
 		      -Wno-format-y2k \