Use gcc warning flag -Wnested-externs

If the compiler supports the warning flag -Wnested-externs, use it.

Avoid the only warning by moving the declaration of xml_builtin to a
more proper place.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -140,7 +140,7 @@ LDFLAGS="-g $LDFLAGS"
 
 gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"
 gcc_flags="-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers $gcc_flags"
-gcc_flags="-Wmissing-include-dirs -Wempty-body $gcc_flags"
+gcc_flags="-Wmissing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"
 gcc_flags="-fstack-protector-all $gcc_flags"
 cat > $TMPC << EOF
 int main(void) { return 0; }

feature_to_c.sh

@@ -63,7 +63,6 @@ for input; do
 done
 
 echo >> $output
-echo "extern const char *const xml_builtin[][2];" >> $output
 echo "const char *const xml_builtin[][2] = {" >> $output
 
 for input; do

gdbstub.c

@@ -1504,7 +1504,6 @@ static int memtox(char *buf, const char *mem, int len)
 
 static const char *get_feature_xml(const char *p, const char **newp)
 {
-    extern const char *const xml_builtin[][2];
     size_t len;
     int i;
     const char *name;

gdbstub.h

@@ -38,4 +38,7 @@ int gdbserver_start(int);
 int gdbserver_start(const char *port);
 #endif
 
+/* in gdbstub-xml.c, generated by feature_to_c.sh */
+extern const char *const xml_builtin[][2];
+
 #endif