utils/mkdir-p: check if dir exists also after mkdir failed

utils/mkdir-p: check if dir exists also after mkdir failed

with "make install -j8" it happens very often that two or more make
instances are creating the same directory in parallel. As a result one
instace creates the directory and second mkdir fails because the
directory exists already (but it did not while testing for it earlier).

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1204)