Regressions in build process introduced since August

[Stefan Weil]

Dear all,

yesterday I tried to build new QEMU installers for Windows and noticed 
two regressions which break my build process:

*** Change in handling of --extra-cflags

Running `configure [...] --extra-cflags="-I /xyz"` results in compiler 
flags `-I [...] /xyz`, so the `-I` and `/xyz` are separated by other 
compiler flags which obviously cannot work as expected. I could work 
around that by removing the space and using a pattern like `-I/xyz`.

This regression is not restricted to builds targeting Windows.

*** Setting INSTALLER no longer handled

meson.build sets a hard name for the Windows installer executable: 
installer = 'qemu-setup-' + meson.project_version() + '.exe'.

Previously the installer name could be changed by running `make 
installer INSTALLER=qemu-setup-something.exe`. This no longer works. Is 
there an alternative solution how the name of the installer executable 
can be set? Or how could I reimplement the lost functionality?

Kind regards

Stefan Weil