Remove -Wdeclaration-after-statement from list of enabled warnings

We use C99 features in various places (especially init-statement in for
loops), and C99 allows declaration after statement. This also avoids
warnings from other projects, like GObject Introspection >= 1.76.1 via
g-ir-scanner.

m4macros/xdt-features.m4

@@ -104,7 +104,6 @@ AS_HELP_STRING([--disable-debug],[Include no debugging support]),
   fi
 
   xdt_cv_additional_CFLAGS="$xdt_cv_additional_COMMON_FLAGS \
-                            -Wdeclaration-after-statement \
                             -Wnested-externs \
                             -Wold-style-definition"
   xdt_cv_additional_CXXFLAGS="$xdt_cv_additional_COMMON_FLAGS"