Allow space in PERL spec (unix only)

Someone wants to configure like this:

    PERL="/usr/bin/env perl" ./config

The end goal is to get that in the #! line of CA.pl and a few other
scripts.  That works well already, but in the Makefile, there were a
few lines looking like this:

    PERL=$(PERL) $(PERL) whatever.pl ...

Those need some quoting.

RT#4311

Reviewed-by: Matt Caswell <matt@openssl.org>