Make implicit declarations and wrong return types hard errors

This makes three warnings hard errors:

1. `implicit-int`
> Warn when a declaration does not specify a type.

2. `implicit-function-declaration`
> Give a warning whenever a function is used before being declared.

3. `return-type`
> Warn whenever a function is defined with a return type that defaults to
> "int".  Also warn about any "return" statement with no return value in
> a function whose return type is not "void" (falling off the end of the
> function body is considered returning without a value).
>
> For C only, warn about a "return" statement with an expression in a
> function whose return type is "void", unless the expression type is
> also "void".

The compiler behaviour when these warnings occur is not the behaviour
the developer expects. So even during development it makes sense that
they are errors.

configure

@@ -4181,6 +4181,123 @@ if test x"$citusac_cv_prog_cc_cflags__Werror_vla" = x"yes"; then
   CITUS_CFLAGS="$CITUS_CFLAGS -Werror=vla"
 fi
   # visual studio does not support these
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -Werror=implicit-int" >&5
+$as_echo_n "checking whether $CC supports -Werror=implicit-int... " >&6; }
+if ${citusac_cv_prog_cc_cflags__Werror_implicit_int+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  citusac_save_CFLAGS=$CFLAGS
+flag=-Werror=implicit-int
+case $flag in -Wno*)
+	 flag=-W$(echo $flag | cut -c 6-)
+esac
+CFLAGS="$citusac_save_CFLAGS $flag"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  citusac_cv_prog_cc_cflags__Werror_implicit_int=yes
+else
+  citusac_cv_prog_cc_cflags__Werror_implicit_int=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$citusac_save_CFLAGS"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $citusac_cv_prog_cc_cflags__Werror_implicit_int" >&5
+$as_echo "$citusac_cv_prog_cc_cflags__Werror_implicit_int" >&6; }
+if test x"$citusac_cv_prog_cc_cflags__Werror_implicit_int" = x"yes"; then
+  CITUS_CFLAGS="$CITUS_CFLAGS -Werror=implicit-int"
+fi
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -Werror=implicit-function-declaration" >&5
+$as_echo_n "checking whether $CC supports -Werror=implicit-function-declaration... " >&6; }
+if ${citusac_cv_prog_cc_cflags__Werror_implicit_function_declaration+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  citusac_save_CFLAGS=$CFLAGS
+flag=-Werror=implicit-function-declaration
+case $flag in -Wno*)
+	 flag=-W$(echo $flag | cut -c 6-)
+esac
+CFLAGS="$citusac_save_CFLAGS $flag"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  citusac_cv_prog_cc_cflags__Werror_implicit_function_declaration=yes
+else
+  citusac_cv_prog_cc_cflags__Werror_implicit_function_declaration=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$citusac_save_CFLAGS"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $citusac_cv_prog_cc_cflags__Werror_implicit_function_declaration" >&5
+$as_echo "$citusac_cv_prog_cc_cflags__Werror_implicit_function_declaration" >&6; }
+if test x"$citusac_cv_prog_cc_cflags__Werror_implicit_function_declaration" = x"yes"; then
+  CITUS_CFLAGS="$CITUS_CFLAGS -Werror=implicit-function-declaration"
+fi
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -Werror=return-type" >&5
+$as_echo_n "checking whether $CC supports -Werror=return-type... " >&6; }
+if ${citusac_cv_prog_cc_cflags__Werror_return_type+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  citusac_save_CFLAGS=$CFLAGS
+flag=-Werror=return-type
+case $flag in -Wno*)
+	 flag=-W$(echo $flag | cut -c 6-)
+esac
+CFLAGS="$citusac_save_CFLAGS $flag"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  citusac_cv_prog_cc_cflags__Werror_return_type=yes
+else
+  citusac_cv_prog_cc_cflags__Werror_return_type=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$citusac_save_CFLAGS"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $citusac_cv_prog_cc_cflags__Werror_return_type" >&5
+$as_echo "$citusac_cv_prog_cc_cflags__Werror_return_type" >&6; }
+if test x"$citusac_cv_prog_cc_cflags__Werror_return_type" = x"yes"; then
+  CITUS_CFLAGS="$CITUS_CFLAGS -Werror=return-type"
+fi
+
 
 #
 # --enable-coverage enables generation of code coverage metrics with gcov

configure.in

@@ -170,6 +170,9 @@ CITUSAC_PROG_CC_CFLAGS_OPT([-Wmissing-declarations])
 CITUSAC_PROG_CC_CFLAGS_OPT([-Wmissing-prototypes])
 CITUSAC_PROG_CC_CFLAGS_OPT([-Wshadow])
 CITUSAC_PROG_CC_CFLAGS_OPT([-Werror=vla])  # visual studio does not support these
+CITUSAC_PROG_CC_CFLAGS_OPT([-Werror=implicit-int])
+CITUSAC_PROG_CC_CFLAGS_OPT([-Werror=implicit-function-declaration])
+CITUSAC_PROG_CC_CFLAGS_OPT([-Werror=return-type])
 
 #
 # --enable-coverage enables generation of code coverage metrics with gcov