Makefile.shared: fix to allow strings and spaces in passed variables

Makefile.shared: fix to allow strings and spaces in passed variables

The previous change for mingw, which now defaults to OPENSSLDIR and
ENGINESDIR definitions that include a space, a long standing issue was
revealed again; our builds for Unix like environment were never very
tolerant of spaces in these definitions, because the quotes were
interpreted along the way.

New analysis of Makefile.shared showed that our use of quotes in there
wasn't quite right.  A lot of double quotes could safely be replaced
with single quotes, thus protecting the diverse values we pass down to
this build file (remember that make variables are expanded before
passing the command to the shell, unconditionally), reserving double
quotes to the places where absolutely needed (to protect the expansion
of shell variables to commands).

CVE-2019-1552

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9469)