malloc: Always install mtrace (bug 31892)

Generation of the Perl script does not depend on Perl, so we can
always install it even if $(PERL) is not set during the build.

Change the malloc/mtrace.pl text substition not to rely on $(PERL).
Instead use PATH at run time to find the Perl interpreter. The Perl
interpreter cannot execute directly a script that starts with
“#! /bin/sh”: it always executes it with /bin/sh.  There is no
perl command line switch to disable this behavior.  Instead, use
the Perl require function to execute the script.  The additional
shift calls remove the “.” shell arguments.  Perl interprets the
“.” as a string concatenation operator, making the expression
syntactically valid.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>