configure: make source tree build more robust

configure: make source tree build more robust

When source directory can be arrived at by two paths,
configure might misdetect an out of tree build.
The simplest way to trigger the problem is running
configure using a full path. E.g. (<firstpath> refers to qemu source
tree):
    ln -s <firstpath> <secondpath>
    cd <firstpath>
    <secondpath>/configure

A more practical way is when make runs configure automatically:

1. cd <firstpath>/; ./configure
    SRC_PATH=<firstpath>/ is written into config_host.mak
2. cd <secondpath>/; touch configure; make
    make now runs <firstpath>/configure, so configure
    assumes it's an out of tree build

When this happens configure overwrites parts of
the current tree with symlinks.

Make the test more robust: look for configure
in the current directory.
If there - we know it's a source build!

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>