BUILD: fix SUBVERS extraction in the Makefile

We'd rather skip any line containing "$Format" and not just those
beginning with it because SUBVERS starts with a dash and caused a
bad format to be reported.
diff --git a/Makefile b/Makefile
index 01e6959..0c36650 100644
--- a/Makefile
+++ b/Makefile
@@ -356,10 +356,10 @@
 VERSION := $(shell cat VERSION 2>/dev/null || touch VERSION)
 endif
 ifeq ($(SUBVERS),)
-SUBVERS := $(shell (grep -v '^\$$' SUBVERS 2>/dev/null || touch SUBVERS) | head -n 1)
+SUBVERS := $(shell (grep -v '\$$Format' SUBVERS 2>/dev/null || touch SUBVERS) | head -n 1)
 endif
 ifeq ($(VERDATE),)
-VERDATE := $(shell (grep -v '^\$$' VERDATE 2>/dev/null || touch VERDATE) | head -n 1 | cut -f1 -d' ' | tr '-' '/')
+VERDATE := $(shell (grep -v '^$$Format' VERDATE 2>/dev/null || touch VERDATE) | head -n 1 | cut -f1 -d' ' | tr '-' '/')
 endif
 
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