configure: Avoid duplicate flags when calling compile_prog

configure: Avoid duplicate flags when calling compile_prog

This patch removes redundant shell code and cleans it a little bit.

Shell macro compile_prog takes two arguments:
local_cflags and local ldflags.

$QEMU_CFLAGS is added automatically to the cflags, so there is no need
to pass it as an argument.

It is also unnecessary to pass -Werror twice.

$flag is a compiler warning option, so it should be in local_cflags
instead of local_ldflags.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>