BUG#57933 "add -Wdeclaration-after-statement to gcc builds";

first part, for autotools build.

config/ac-macros/maintainer.m4:
  Add the flag. With it, and as we use -Werror, we nicely get
  "error: ISO C90 forbids mixed declarations and code" if
  a declaration follows a statement in C code.
  Note that g++ refuses this flag.

config/ac-macros/maintainer.m4

@@ -19,6 +19,7 @@ AC_DEFUN([MY_MAINTAINER_MODE_WARNINGS], [
   AS_IF([test "$GCC" = "yes"], [
     C_WARNINGS="-Wall -Wextra -Wunused -Wwrite-strings -Wno-strict-aliasing -Werror"
     CXX_WARNINGS="${C_WARNINGS} -Wno-unused-parameter"
+    C_WARNINGS="${C_WARNINGS} -Wdeclaration-after-statement"
   ])
 
   # Test whether the warning options work.