configure.ac: if neither UPSCONN{,_t} type was found, refuse to build NUT plugin

configure.ac: if neither UPSCONN{,_t} type was found, refuse to build NUT plugin

NOTE: src/nut.c also has pragmas to error out in this situation,
but that handling is compiler-dependent and happens too late in
the checkout/configure/build loop.

Presumably this inability to find the type in the earlier-found header file
is also triggered by build environment "inconsistencies" like lack of basic
types in the libc implementation (maybe highlighting the need for additional
headers or macros for the platform).