configure script breaks when $source_path contains white spaces

Hi,

I noticed that the configure script breaks when the qemu source directory is in a path containing white spaces, in particular the list of targets is not correctly generated when calling "./configure --help".

Steps to reproduce the problem:

$ mkdir "dir with spaces"
$ cd dir\ with\ spaces/
$ git clone https://git.qemu.org/git/qemu.git
$ cd qemu/
$ ./configure --help | grep -A3 target-list


Actual result:

  --target-list=LIST       set target list (default: build everything)
                           Available targets: dir with *-softmmu dir with 
                           *-linux-user


Expected result:

  --target-list=LIST       set target list (default: build everything)
                           Available targets: aarch64-softmmu alpha-softmmu 
                           arm-softmmu cris-softmmu hppa-softmmu i386-softmmu 
                           lm32-softmmu m68k-softmmu microblaze-softmmu 


This happens because the $mak_wilds variable uses spaces to separate different paths, maybe newlines may be used, which are less likely to be in directory names.

BTW "shellcheck" may help finding some other problems.

Qemu version:

$ git describe 
v3.1.0-1960-ga05838cb2a

Thanks,
   Antonio