configure.in: Set CPPFLAGS when checking headers.

configure.in: Set CPPFLAGS when checking headers.

When checking for <upsclient.h> (and the headers of a few other libraries) the
library specific CFLAGS were added to the global CFLAGS variable. This variable
is used when checking for ``header usability'', but not when checking for
``header presence'' for some reason.

This commit uses the global CPPFLAGS variable instead, which is used in both
checks. Libraries that benefit from this commit are:
- libcurl,
- libmysql(client),
- libnetsnmp,
- libupsclient.

Resolves #6