Let the user define YYYYMMDD

This is very useful when debugging a build from a previous day.

Author: tuliom

Makefile

@@ -6,7 +6,7 @@ MOCK_CHROOT?=fedora-rawhide-x86_64
 MOCK_OPTS?=
 MOCK_OPTS_RELEASE?=--no-clean --no-cleanup-after $(MOCK_OPTS)
 MOCK_OPTS_SNAPSHOT?=$(MOCK_OPTS_RELEASE) --with snapshot_build $(MOCK_OPTS)
-YYYYMMDD=$(shell date +%Y%m%d)
+YYYYMMDD?=$(shell date +%Y%m%d)
 SOURCEDIR=$(shell pwd)
 SPEC=llvm.spec
 # When nothing is given, this will be determined based on