package/clamav: fix build with uClibc

clamav wants to use backtrace, and decides whether it can use it if it
detects a glibc >= 2.1.

But uClibc does impersonate a glibc >= 2.1, so clamav concludes it is
possible to use backtrace. So it includes execinfo.h, which is missing
in our default uClibc config file.

So, just extend the test so that backtrace support is disable on uClibc,
unless it has been configured with backtrace support.

A far better solution would be to add a ./configure check for backtrace,
but this patch is sufficient enough.

Fixes:
    http://autobuild.buildroot.net/results/cff/cffa32fcedda735983d4805d6d4fa77844539b10/
    http://autobuild.buildroot.net/results/e0a/e0a765a94a538b0b936ea512f7aba0264fac6309/
    ...

Bugtracker: https://bugzilla.clamav.net/show_bug.cgi?id=11170



[Peter: add bugtracker URL as suggested by Bernd]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>