config.h can't be prefixed unconditionally

The script make-event-config.sed was mangling all the symbols by
prefixing them with "EVENT__".  The problem here is that some
symbols aren't for local consumption within libevent, but rather
influence other system header files (ex: __USE_FILE_OFFSET64 is
used by dozens of header files including <sys/sendfile.h>).

As a workaround, all symbols starting with a capital letter only
(with the exception of STDC_HEADERS which must also be left
untouched) will be mangled.

Future contributors will need to be aware of this distinction.

make-event-config.sed

@@ -18,6 +18,10 @@ $a\
 \
 #endif /* event2/event-config.h */
 
-s/#\( *\)define /#\1define EVENT__/
-s/#\( *\)undef /#\1undef EVENT__/
-s/#\( *\)if\(n*\)def /#\1if\2def EVENT__/
+/#\( *\)undef STDC_HEADERS\>/b
+/#\( *\)define STDC_HEADERS\>/b
+
+# Only rewrite symbols starting with capitals
+s/#\( *\)define \([A-Z]\)/#\1define EVENT__\2/
+s/#\( *\)undef \([A-Z]\)/#\1undef EVENT__\2/
+s/#\( *\)if\(n*\)def \([A-Z]\)/#\1if\2def EVENT__\2/