Makefile: use overridable $(FIND) instead of hard-coded 'find'

The Makefile already offers the variable $(FIND) and uses it except in one
place. Fix it.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2093,7 +2093,7 @@ $(patsubst %.perl,%,$(SCRIPT_PERL)): perl/perl.mak
 perl/perl.mak: perl/PM.stamp
 
 perl/PM.stamp: FORCE
-	$(QUIET_GEN)find perl -type f -name '*.pm' | sort >$@+ && \
+	$(QUIET_GEN)$(FIND) perl -type f -name '*.pm' | sort >$@+ && \
 	{ cmp $@+ $@ >/dev/null 2>/dev/null || mv $@+ $@; } && \
 	$(RM) $@+