build system: Several fixes for libcurl, libmysql, and libupsclient.

build system: Several fixes for libcurl, libmysql, and libupsclient.

libcurl:      Check for CFLAGS using `curl-config --cflags' and use these
	      CFLAGS when checking for <curl/curl.h> to allow for weird
	      locations of the library.
libmysql:     Cleaned up the checking code.
libupsclient: Check if the header file provides `UPSCONN' or `UPSCONN_t' and
	      use the one available. This should fix compile issues with with
	      the nut plugin.