configure: Fix creation of symbolic links for MinGW toolchain

configure: Fix creation of symbolic links for MinGW toolchain

The MinGW toolchain on w32/w64 hosts does not create symbolic links,
but implements 'ln -s' similar to 'cp -r'.

In incremental out of tree builds, this resulted in files which
were not updated when their counterparts in the QEMU source tree
changed. Especially for Makefile* this happened very often.

With this patch, the 'symlinked' files are now always updated for
out of tree builds. Similar code was already used for the symbolic
link of libcacard/Makefile.

The symlink macro always removes the target before it is created
again, therefore the rm command for libcacard/Makefile was redundant
and is removed now.

Macro symlink is also used with directories. To remove them on w32
hosts, a recursive rm is needed.

v2:
Quote arguments in shell function symlink, and also quote any argument
which is passed to symlink and which contains macros. This should reduce
the chance of accidents caused by rm -rf.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>