config.h can't be prefixed unconditionally

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.