Avoid potential conflicts between #defines in opensslconf.h and

Avoid potential conflicts between #defines in opensslconf.h and
defines when compiling applications, and allow applications to
select what #defines to enable -- OPENSSL_EXLUCDE_DEFINES
enables the "#define NO_whatever" stuff only, which avoids
potential severe confusion caused by "#define _REENTRANT" when
opensslconf.h is not the first header file #included.