[xmlsec] build config change for nss

[aleksey at aleksey.com]

> For the Debian package I've had to change the pkg-config mozilla calls
> to xulrunner...

Oh... Daniel also had to tweak NSPR/NSS package names for Fedora... I just
updated xmlsec configuration scripts and I believe it should handle new
NSPR/NSS packages' names for both Fedora and Debian.

Jon, Daniel, can you try the current CVS version, please? If everything works
for you then I will try (very hard) to do an xmlsec release next week.

Thank you in advance,
Aleksey