configure: Don't run configure tests with -Werror enabled

Don't run configure tests with -Werror in the compiler flags. The idea
of -Werror is that it makes problems very obvious to developers, so
they get fixed quickly. However, when running configure tests, failures
due to -Werror are far from obvious -- they simply result in the test
quietly failing when it should have passed. Not using -Werror is in
line with recommended practice in the Autoconf world.

This commit is essentially backing out the changes in commit 417c9d72.
Instead we fix the problem that commit was trying to address in a
different way: we add -Werror only for the test of the nss headers,
with a comment that this is specifically intended to detect a bug
in some releases of nss.

We also have to clean up a bug in the smartcard test where it was
trying to include smartcard_cflags in the test compile flags: this
would always result in a failure with -Werror, because they include
an escaped "$(SRC_PATH)" which is only valid when used in the final
makefile.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
