Use compat mkdtemp() on Solaris boxes

Use compat mkdtemp() on Solaris boxes

Define NO_MKDTEMP for all variants of SunOS; Solaris 10 does not
have mkdtemp() and all the other versions our Makefile knows
about don't have it either.

NO_{SETENV,UNSETENV,C99_FORMAT,STRTOUMAX} definitions cannot be
unified across versions.  Beginning with Solaris 10, the C-library
provides unsetenv(), setenv() and strtoumax().  Also 'z'/'t' formats
are supported.  However, older versions of Solaris do not support
these.

Signed-off-by: Guido Ostkamp <git@ostkamp.fastmail.fm>
Signed-off-by: Junio C Hamano <gitster@pobox.com>