meson: Improve cross-compiling support on Windows
This MR fixes two issues happening when cross-compiling mesa on Windows (targeting an Unix device):
- Although the flex/win_flex tool is run on the build system, the files it generates are compiled for the host system, so the
--wincompat
flags must be added only when the target is Windows - The nm/dumpbin program is run on the build system, not the host system. So the check must be done accordingly.
This MR, along with another unrelated MR which I'll do later, allows the Android turnip driver to be compiled on Windows.