rework check_prog to improve for " " in prog path

Most MSWin32 systems have "C:\Program Files...\bin\gnuprog.exe" - deal
with that even on simple progs.
re-enable cleanup at END
don't TODO -lm on non-win32
add nytprof stuff being ignored
doing some micro-optimizations

even if longest part is invoking cc, it doesn't need wasting time
note Changes for 0.304