build: add "-Wvla" to default CFLAGS

Variable length arrays silently break _NL_STATIC_ASSERT(), if the
argument of the assertion is not a constant. Avoid that pitfall, by not
allowing VLAs.

That was already the case earlier, and the github CI checked for that.
Now also add "-Wvla" to the default flags.

Makefile.am

@@ -35,11 +35,14 @@ pkgconfig_DATA = \
 warn_cppflags = \
 	-Wall \
 	-Wextra \
+	\
 	-Wmissing-prototypes \
-	-Wno-unused-parameter \
-	-Wno-sign-compare \
-	-Wno-missing-field-initializers \
 	-Wpointer-arith \
+	-Wvla \
+	\
+	-Wno-missing-field-initializers \
+	-Wno-sign-compare \
+	-Wno-unused-parameter \
 	$(NULL)
 
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