configure: fix double check tests with Clang

configure: fix double check tests with Clang

Configuring with Clang compiler with -Werror would not work after
improved checks:
/tmp/qemu-conf--25992-.c:4:32: error: self-comparison always evaluates
to true [-Werror,-Wtautological-compare]
int main(void) { return preadv == preadv; }
/tmp/qemu-conf--25992-.c:13:26: error: self-comparison always
evaluates to true [-Werror,-Wtautological-compare]
    return epoll_create1 == epoll_create1;
/tmp/qemu-conf--25992-.c:3:13: error: explicitly assigning a variable
of type 'char **' to itself [-Werror,-Wself-assign]
    environ = environ;

Avoid the errors by adjusting the tests.

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