GNU Make 4.4 reports undefined variable warnings about MERC_INT_DIR.
The problem is related to MERC_INT_DIR being referenced in a
$(shell ...) command, but being defined later.
scripts/Mmake.vars.in:
Move the definition of MERC_INT_DIR to Mmake.vars,
before its reference in MERC_INT_DIR_FOR_VPATH.
scripts/Mmake.rules:
Delete definition of MERC_INT_DIR from Mmake.rules.