Fix an issue exposed by commit 7d001786a0.
VPATH indirectly refers to GRADESTRING_NO_TARGET.
When the VPATH is being calculated after the makefile is read,
the reference to GRADESTRING_NO_TARGET causes make to try to run the
Mercury compiler to output a grade string, but we may not yet have a
working Mercury compiler.
scripts/Mmake.vars.in:
Set EXTRA_INIT_DIRS_NO_TARGET to a non-empty string
(that references GRADESTRING_NO_TARGET in a pattern substitution)
only if EXTRA_LIB_DIRS is actually non-empty.