`cmd.exe` built-in `cd` does not recognize slashes as path separators,
replace to backslashes.

The clean targets should clean up all extensions, including those have
nothing to build for any reason, e.g., platform requirements etc.

When multiple libraries exist in a subdirectory under `ext`, `rmdir
-p` may fail, because other directories have not been removed yet or
the parent directory has been removed by other `distclean`.  `rmdir`
in GNU coreutils has `--ignore-fail-on-non-empty` option for the
former case but others may not, and the latter race condition is not
avoidable anyway.

The condition is rarely met in usual cases, so it's worth to note about
the situation and the reason why we care about it.

To strip enclosing double quotes.

exts.mk files are one level under the top of extension directories.

This is another attempt to fix out-of-src build with
--with-static-linked-ext. The first attempt was
4f1888bda70981d9f5b1bf55ab692e0ce18e79f4 but reverted because it broke
out-of-src build from pre-generated sources via `make dist`.
This patch fixes the second trans C source gen, mentioned in the
previous commit message, by passing MINIRUBY as well as when invoking
from common.mk