We need to force an update to Makefile (not Makefile.PL) when
MANIFEST changes. Since "Makefile" (aka. "$(FIRST_MAKEFILE)")
is already a single-colon make target; we can't create a
double-colon rule to augment it. So we'll continue using a
"Makefile.PL" rule, but have it recreate the resulting Makefile
Finally, change the "check" target to use "prove -b" instead of
"prove -l" so we test against "blib/lib", since what's in the
"blib" dir will be installed.
Fixes: 4c20de0694d06ff3 ("Makefile.PL: add dependency on MANIFEST contents")