Fix debug_defines make process.

With the old version I ended up with a debug_defines.h that had many
copies of itself. Have registers.py always generate the whole thing, so
there is no chance of repeated executions simply appending again and
again.

Makefile

@@ -62,15 +62,13 @@ changelog.tex: .git/logs/HEAD Makefile
 	git log --no-merges --date=short --pretty="format:vhEntry{%h}{%ad}{%an}{%s}" | \
 	    sed -e "s,\\\\,{\\\\textbackslash},g" -e "s,[_#^],\\\\&,g" -e s/^/\\\\/ >> changelog.tex
 
-debug_defines: debug_defines.h debug_defines.c $(patsubst %,xml/%,$(REGISTERS_TEX:.tex=.xml)) registers.py
-	./registers.py $@ --cgetters $(filter %.xml, $^)
-
-debug_defines.%:
-	echo "/*" > $@
-	echo " * This file is auto-generated by running 'make debug_defines' in" >> $@
-	echo " * https://github.com/riscv/riscv-debug-spec/ (`git describe --always --dirty --exclude '*'`)" >> $@
-	echo " */" >> $@
-	echo >> $@
+.PHONY: debug_defines
+debug_defines: debug_defines.h debug_defines.c
+
+debug_defines.c: debug_defines.h
+
+debug_defines.h: $(patsubst %,xml/%,$(REGISTERS_TEX:.tex=.xml)) registers.py
+	./registers.py debug_defines --cgetters $(filter %.xml, $^)
 
 %.eps: %.dot
 	dot -Teps $< -o $@

registers.py

@@ -11,6 +11,7 @@
 import operator
 from functools import cmp_to_key
 from functools import reduce
+import subprocess
 
 class Registers( object ):
     def __init__( self, name, label, prefix, description, skip_index,
@@ -810,9 +811,19 @@ def main():
     parsed = parser.parse_args()
 
     if (parsed.xml_paths):
-        fd_h = open( parsed.path + ".h", "a" )
+        git_version = subprocess.run("git describe --always --dirty --exclude *".split(),
+                                    capture_output=True, check=True, text=True).stdout.strip()
+        header = ("/*\n"
+            " * This file is auto-generated by running 'make debug_defines' in\n"
+            f" * https://github.com/riscv/riscv-debug-spec/ ({git_version})\n"
+            " */\n"
+            "\n")
+
+        fd_h = open( parsed.path + ".h", "w" )
+        fd_h.write(header)
         fd_h.write("#ifndef DEBUG_DEFINES_H\n#define DEBUG_DEFINES_H\n")
-        fd_c = open( parsed.path + ".c", "a" )
+        fd_c = open( parsed.path + ".c", "w" )
+        fd_c.write(header)
         fd_c.write(f'#include "{parsed.path}.h"\n#include <stddef.h>\n#include <assert.h>\n')
         registers_list = [parse_xml( xml_path ) for xml_path in parsed.xml_paths]
         for registers in registers_list: